Learn Hardcore Turkish: Telefonun yine sessizde olduğunu fark ettim. - I noticed that your phone is on silent again.

Because -un is both the genitive of telefon and the possessive suffix “your” on telefon, context will decide whether you read it as the phone or your phone (see the next question).

Is this talking about the phone or your phone? How can I tell from telefonun?

Formally, telefonun is ambiguous:

telefon
- genitive = telefonun → of the phone / the phone’s
senin telefonun = your phone (you can drop senin and just say telefonun)

So:

Telefonun sessizde.
→ most naturally: Your phone is on silent.

But in -DIK clauses (like …olduğunu), the subject of the embedded clause is usually in genitive:

Telefonun sessizde olduğunu fark ettim.
→ I realized that the phone was on silent.

If a possessor is also involved (e.g. your phone as the subject), you normally add an extra genitive ending:

Senin telefonun sessizde. (Your phone is on silent.)
Senin telefonunun sessizde olduğunu fark ettim.
→ I realized that your phone was on silent.

So:

Telefonun yine sessizde olduğunu fark ettim.
by default = I realized that the phone was on silent again.

To clearly say your phone as the subject of that clause, you’d normally say:

Telefonunun yine sessizde olduğunu fark ettim.

Why is it sessizde with -de, and not just sessiz?

Sessiz = silent (adjective).

Sessiz-de = in (the state of) silent → essentially “on silent” as a phone setting.

Turkish often uses the locative case -de/-da to talk about modes or states, especially with devices:

Telefon sessizde. – The phone is on silent.
Telefon uçak modunda. – The phone is in flight mode.
Bilgisayar kapalıydı. – The computer was off. (here -dı is just past, but you can also say kapalıydı = in the off state)

So sessizde olmak ≈ to be on silent (mode), more idiomatic than just sessiz olmak for a phone.

You could also say:

Telefon sessiz moddaydı. – The phone was in silent mode.

But telefon sessizde is shorter and very common.

What is inside olduğunu? Which suffixes are used here?

Olduğunu is a packed form. Morphologically:

ol- = root of olmak (to be / to become)
-duğ- = the -DIK / -DUK / -DÜK / -DIĞ / -DUĞ / -DÜĞ nominalizing suffix
-u = 3rd person singular possessive (its)
-nu = -n (buffer consonant) + -u (accusative case)

Step by step:

ol- + -duğ- + -u → olduğu
- roughly “its being / the fact that it is/was”
Add accusative -(y)ı / -i / -u / -ü. Because of 3rd person possessive, we insert -n-:
- olduğu + -nu → olduğunu

So olduğunu = its being (in accusative case).
The whole object is:

telefonun yine sessizde olduğunu
= the phone’s (being) on silent again
= “that the phone was on silent again.”

Why do we need olduğunu here? Could I say Telefon yine sessizdeydi fark ettim instead?

There are two different structures:

With a subordinate clause (what the Turkish sentence uses):
- Telefonun yine sessizde olduğunu fark ettim.
  → I realized that the phone was on silent again.
Here, telefonun yine sessizde olduğunu is a nominalized clause (a noun phrase) and is the direct object of fark ettim.
With two separate clauses (what you suggested):
- Telefon yine sessizdeydi, fark ettim.
  Literally: The phone was on silent again, I realized.
This is grammatical, but it sounds like two statements put side by side:
- first you state a fact: The phone was on silent again.
- then you add: I realized.

The Turkish sentence you’re analyzing is a single sentence with an explicit “that”-clause as its object. For that, you basically have to use:

[subject in genitive] + [predicate + -DIK + possessive] + (case)
here: telefonun … sessizde olduğunu

So:

If you want “I realized that …”, you need some form like …olduğunu fark ettim.
If you just put sessizdeydi before fark ettim, it becomes a looser juxtaposition, not a tightly embedded “that”-clause.

Is olduğunu past tense because of -du-?

Not exactly. In olduğunu, -duğ- comes from the -DIK nominalizer, not the simple past tense -DI.

-DI (past tense on finite verbs):

ol-du – it became / it was (simple past, finite verb)

-DIK (nominalizer on verb stems):

ol-duğ-u – its being / the fact that it is/was (non‑finite, behaves like a noun)

The tense meaning of a -DIK clause is relative to the main verb and the context:

Telefonun sessizde olduğunu biliyorum.
→ I know that the phone is on silent. (present meaning)
Telefonun sessizde olduğunu fark ettim.
→ I realized that the phone was on silent. (past situation realized in the past)

So:

The past meaning here mainly comes from fark ettim (I noticed / realized – clear past).
olduğunu itself doesn’t by default mean “had been” or “was”; it’s a nominalized “(its) being” whose time reference depends on the matrix verb.

Why is olduğunu in the accusative case? Do -DIK clauses always get -ı / -i / -u / -ü?

Olduğunu is in accusative because, as a whole, it is the direct object of fark ettim.

In Turkish:

A that‑clause (like “that the phone was on silent”) behaves like a noun phrase.
When such a clause is a definite / specific object, it is normally marked with the accusative:

Examples:

Onun geldiğini gördüm.
→ I saw that he came / I saw him come.
(geldiğini = his having come, in accusative)
Senin hasta olduğunu biliyorum.
→ I know that you are sick.

Same pattern here:

Telefonun yine sessizde olduğunu fark ettim.
→ I realized that the phone was on silent again.

So: -ı / -i / -u / -ü is not automatic with -DIK clauses, but when the clause is a direct object and is definite (which object clauses almost always are), you normally see accusative on the -DIK form.

Why is fark ettim two words instead of farkettim?

Because fark etmek is a compound verb consisting of:

fark – a noun: difference
etmek – a light verb: to do / to make

Together they mean “to notice / to realize”.

In standard spelling, such combinations with etmek, olmak, kılmak, etc., are normally written as two words:

fark etmek → to notice
- fark ettim – I noticed / realized
yardım etmek → to help
- yardım ettim – I helped
namaz kılmak → to perform prayer
- namaz kıldım – I prayed (Muslim ritual prayer)

Writing farkettim is common informally (especially online), but fark ettim is the correct standard spelling.

Can the word order be different, like Yine telefonun sessizde olduğunu fark ettim or Telefonun olduğunu yine sessizde fark ettim?

Some changes are fine, others become odd or change the meaning.

Yine telefonun sessizde olduğunu fark ettim.
- Here yine is moved left.
- Now it more naturally modifies fark ettim:
  “I again realized that the phone was on silent.”
  (i.e. I realized it once before, and I realized it again.)
In the original:
- Telefonun yine sessizde olduğunu fark ettim.
  yine is next to sessizde, so it sounds like:
  - the phone was on silent again (the setting was repeated),
    not I realized again.
Telefonun olduğunu yine sessizde fark ettim.
- This is not natural.
- Inside the embedded clause, you are splitting things in an odd way:
  - telefonun olduğunu – “that it was the phone”
  - yine sessizde – “again on silent”
- It sounds like: “I noticed in silent mode again that it was the phone,” which doesn’t make semantic sense in context.

Natural patterns:

[Subject in genitive] [adverb] [state] [olduğunu] [main verb]
→ Telefonun yine sessizde olduğunu fark ettim.
If you want yine to modify fark ettim (again I realized), you can move it:
→ Yine telefonun sessizde olduğunu fark ettim.

What is the difference between yine and gene? Could I say Telefonun gene sessizde olduğunu fark ettim?

Yes, you can say:

Telefonun gene sessizde olduğunu fark ettim.

Yine and gene:

They are near-synonyms, both meaning roughly “again” / “still”.
Yine is:
- a bit more neutral / standard, common in both speech and writing.
Gene is:
- very common in everyday speech,
- sometimes feels a bit more colloquial, depending on context.

In this sentence, yine and gene are fully interchangeable in meaning:

Telefonun yine sessizde olduğunu fark ettim.
Telefonun gene sessizde olduğunu fark ettim.

Both: I realized that the phone was on silent again.

Can we leave telefonun out and just say Yine sessizde olduğunu fark ettim? Would people understand?

Yes, it’s possible if the subject is clear from context.

Yine sessizde olduğunu fark ettim.
→ I realized (that it) was on silent again.

Here, the subject of the embedded clause (the thing that is on silent) is implicit:

In a conversation clearly about your phone, this will naturally be understood as:
- I realized (the phone) was on silent again.

Turkish often omits subjects when they are obvious from context:

Geldiğini duydum. – I heard that (he/she) came.
Hasta olduğunu biliyorum. – I know that (you/he/she) are/is sick.

So:

Telefonun yine sessizde olduğunu fark ettim.
– explicit subject (the phone).
Yine sessizde olduğunu fark ettim.
– subject dropped, inferred from context.

How would the sentence change for my phone, your phone, his/her phone, etc.?

We change the possessor of telefon and then put that possessed noun into genitive for the embedded clause’s subject.

Base frame:

[X’in] [telefon-X-pos]‑un yine sessizde olduğunu fark ettim.

Examples (with possessive pronouns kept for clarity):

My phone
- Benim telefonum sessizde. – My phone is on silent.
- Benim telefonumun yine sessizde olduğunu fark ettim.
  → I realized that my phone was on silent again.
Your phone (singular “you”)
- Senin telefonun sessizde. – Your phone is on silent.
- Senin telefonunun yine sessizde olduğunu fark ettim.
  → I realized that your phone was on silent again.
His / her phone
- Onun telefonu sessizde. – His/her phone is on silent.
- Onun telefonunun yine sessizde olduğunu fark ettim.
  → I realized that his/her phone was on silent again.
Our phone (for example, the family phone)
- Bizim telefonumuz sessizde. – Our phone is on silent.
- Bizim telefonumuzun yine sessizde olduğunu fark ettim.
  → I realized that our phone was on silent again.

In fluent conversation, speakers often drop the pronoun (benim, senin, onun, bizim…) if it’s clear, and you’re left with:

Telefonumun yine sessizde olduğunu fark ettim. – my phone
Telefonunun yine sessizde olduğunu fark ettim. – your / his / her phone (ambiguous without context)

Notice the pattern:

Possessed noun: telefonum, telefonun, telefonu, telefonumuz…
Genitive form for subject position in -DIK clause:
telefonumun, telefonunun, telefonunun, telefonumuzun…

Is there a difference between fark etmek and farkına varmak in a sentence like this?

Both can be used and are close in meaning, but there are nuances:

fark etmek
- very common, somewhat more neutral and direct, like English “to notice / realize”.
- Telefonun yine sessizde olduğunu fark ettim.
  → I noticed / realized that the phone was on silent again.
farkına varmak
- literally “to come to the awareness of”, a bit more formal / literary in some contexts, or it can sound slightly more gradual.
- Telefonun yine sessizde olduğunun farkına vardım.
  - note the genitive on olduğunun (because it’s now complement of farkına)
    → I became aware that the phone was on silent again.

Both are correct; fark ettim is the more straightforward everyday choice here.