Onun sessiz kaldığını ben de varsaydım.

Yes:

• onun = his / her / its
• sessiz = silent / quiet
• kaldığını = that he/she remained / stayed
• ben de = I too / I also
• varsaydım = I assumed

A very important point is that onun sessiz kaldığını is not just three separate words in meaning. It works as one clause-like noun phrase: that he/she remained silent. That whole chunk is the object of varsaydım.

It can mean either, depending on context.

Turkish third-person singular does not mark gender:

• o = he / she / it
• onun = his / her / its

So from onun alone, you cannot know whether the person is male or female. Context tells you.

Because in this kind of embedded clause, Turkish usually puts the subject in the genitive case.

Compare:

• O sessiz kaldı. = He/She remained silent.
• Onun sessiz kaldığını... = that he/she remained silent...

This is a very common Turkish pattern:

• subject of the embedded clause → genitive
• verb of the embedded clause → a nominalized form

So onun is not random here; it is required by the structure.

It is a nominalized verb form built from kalmak (to remain / stay).

Roughly:

• kal- = verb root
• -dık / -dik / -duk / -dük = nominalizing suffix
• -ı / -i / -u / -ü = 3rd-person possessive
• -nı / -ni / -nu / -nü = accusative

So:

• kalmak
• → kaldığı = his/her remaining or that he/she remained
• → kaldığını = that same form in the accusative, because it is the object of varsaydım

The k becoming ğ is a normal sound change between vowels.

There is no separate word here for that.

Instead, Turkish often uses a nominalized clause. So:

• onun sessiz kaldığını

functions like English:

• that he/she remained silent

This is one of the biggest differences between English and Turkish sentence structure. English often uses that; Turkish often uses a verb form like -DIK plus possessive endings.

Because sessiz kalmak is a very natural expression meaning to remain silent or to stay quiet.

It is slightly different from similar expressions:

• sessiz olmak = to be silent
  More stative: describing a condition.
• susmak = to become silent / stop talking
  Often focuses on the moment of going silent.
• sessiz kalmak = to remain/stay silent
  Often emphasizes continuing in that state.

So sessiz kaldığını fits well if the idea is that the person stayed silent.

Ben de means I too or I also.

The word de here is the additive particle also / too, and it is written separately:

• ben de = I too
• o da = he/she too

This is different from the locative suffix -de / -da meaning in / at / on, which is written attached:

• evde = at home

So:

• ben de = separate particle, also
• evde = attached suffix, in/at

Yes, ben could be omitted, because varsaydım already means I assumed.

So:

• Onun sessiz kaldığını varsaydım. = I assumed that he/she remained silent.

However, the original sentence has ben de, which adds a specific meaning: I also assumed it.

That matters, because if you remove ben, you may lose the also meaning or change its scope.

Compare:

• Onun sessiz kaldığını ben de varsaydım.
  = I also assumed that he/she remained silent.
• Onun sessiz kaldığını varsaydım.
  = I assumed that he/she remained silent.
• Onun sessiz kaldığını da varsaydım.
  = I assumed that too.

That last one is different: there, de applies to the whole object clause, not specifically to I.

Yes, it could be changed. Turkish word order is flexible.

The sentence as given:

• Onun sessiz kaldığını ben de varsaydım.

is perfectly natural. It puts the clause onun sessiz kaldığını first, which can help foreground what was assumed.

Another very natural version is:

• Ben de onun sessiz kaldığını varsaydım.

Both are good. The difference is mostly emphasis and information flow.

In general, Turkish often places the main verb at the end, but other parts can move around depending on what the speaker wants to highlight.

Here, varsaydım is the simple past, first person singular of varsaymak (to assume / suppose).

Formation:

• varsay- = verb stem
• -dı = past tense
• -m = I

So:

• varsaydım = I assumed

It is a straightforward finite verb form in the main clause.

Not exactly.

They overlap, but the nuance is different:

• varsaymak = to assume / suppose
  Often sounds more deliberate, logical, or formal.
• sanmak = to think / suppose / believe
  Very common in everyday speech; can imply a mistaken belief.
• düşünmek = to think
  Broader and less specifically assume.

So varsaydım is a good choice if the idea is I made that assumption rather than simply I thought so.

Because it is the direct object of varsaydım.

In Turkish, when a whole clause functions as the object of a verb, it can take accusative marking just like a noun phrase.

So in this sentence:

• main verb = varsaydım
• object = onun sessiz kaldığını

That final -nı on kaldığını is the accusative marker attached to the nominalized clause.

This is another sign that Turkish is treating that he/she remained silent as a noun-like object.