Learn Hardcore Greek: Σήμερα γράφω τις σκέψεις μου στο ημερολόγιό μου πριν κοιμηθώ. - Today I write my thoughts in my diary before I go to sleep.

γράφω ends in -ω, which clearly marks 1st person singular = I write.
So Σήμερα γράφω… already means Today I write….
You would add εγώ only for emphasis or contrast:
Σήμερα εγώ γράφω τις σκέψεις μου… = Today *I (as opposed to someone else) write my thoughts…*

What does τις σκέψεις μου literally mean, and why is there a τις even though English just says “my thoughts”?

Literally, τις σκέψεις μου is:

τις = the (feminine, plural, accusative)
σκέψεις = thoughts
μου = my (unstressed possessive pronoun, literally “of me”)

So word‑for‑word it is “the thoughts of me”, which in natural English is just “my thoughts”.

Greek almost always uses the definite article with a noun + possessive pronoun:

τις σκέψεις μου = my thoughts
το βιβλίο μου = my book
η μητέρα μου = my mother

Where English often drops “the”, Greek keeps it.

Why is the article τις used here and not τα or τους?

The article must agree with the noun σκέψεις in:

gender: feminine
number: plural
case: accusative (it’s the direct object of γράφω)

The feminine plural accusative form of the definite article is τις.

Very briefly:

Masculine plural accusative: τους (π.χ. βλέπω τους φίλους)
Feminine plural accusative: τις (π.χ. βλέπω τις φίλες)
Neuter plural nominative/accusative: τα (π.χ. βλέπω τα βιβλία)

Since σκέψη is feminine, its plural σκέψεις takes τις in the accusative: τις σκέψεις.

Why does the possessive μου come after the noun (σκέψεις μου) instead of before it, like in English?

The usual, unstressed possessive pronouns in Greek (μου, σου, του, της, μας, σας, τους) are enclitics: they normally come after the noun:

οι σκέψεις μου = my thoughts
το ημερολόγιό μου = my diary
η μητέρα σου = your mother

Putting μου before the noun (μου σκέψεις) is wrong in standard Greek.
If you want to emphasize whose something is, you use the special forms δικός μου, δική μου, δικό μου etc., which do go before or around the noun:

οι δικές μου σκέψεις = my (own) thoughts

Do I really need to say μου twice? Why τις σκέψεις μου and also στο ημερολόγιό μου?

Yes, if you want to say “my thoughts in my diary”, you normally mark possession on each noun:

τις σκέψεις μου = my thoughts
στο ημερολόγιό μου = in my diary

If you drop one μου, you change the meaning or make it unclear:

τις σκέψεις στο ημερολόγιό μου
= the thoughts in my diary (they might be mine, but it’s not explicitly said they are my thoughts; the only thing that’s clearly “mine” is the diary)
τις σκέψεις μου στο ημερολόγιο
= my thoughts in the diary (some diary, not specified as mine)

So the original sentence explicitly says that both the thoughts and the diary belong to the speaker.

What exactly is στο in στο ημερολόγιό μου?

στο is a contracted form of the preposition σε plus the definite article το:

σε = in, at, to (depends on context)
το = the (neuter singular)

So:

σε + το → στο

Similarly:

σε + τον → στον (στον φίλο μου = to my friend)
σε + την → στην / στη (στην πόλη = in the city)
σε + τους → στους (στους φίλους μου = to my friends)

Here, στο ημερολόγιό μου literally means “in/to the diary of mine”, i.e. “in my diary”.

Why is it στο ημερολόγιό μου and not just στο ημερολόγιο μου without the extra accent?

The basic form of the word is ημερολόγιο, stressed on the third syllable from the end: η‑με‑ρο‑λό‑γι‑ο.

When this kind of word is followed by an enclitic like μου, Greek treats the combination as one phonological unit. The stress is not allowed to fall earlier than the third syllable from the end of the whole group, so the accent has to move one syllable to the right:

Alone: ημερολόγιο
With μου: στο ημερολόγιό μου

So you see the accent on -γιό- instead of -λό-.

Writing στο ημερολόγιο μου (without moving the accent) is considered a spelling mistake in standard Greek.

Why is γράφω in the present tense? Does it mean “I am writing right now” or something else?

The Greek present tense (here γράφω) covers both:

present progressive: I am writing (right now)
habitual/general present: I write (as a regular action)

With σήμερα, there are two natural readings:

Right now / around now:
Today I’m writing my thoughts in my diary before I go to sleep.
(spoken while or just before doing it)
Routine for today:
Today I (will) write my thoughts in my diary before I go to sleep.
(a plan or typical action for today)

You could also say:

Σήμερα θα γράψω… = Today I will write… (more clearly future, one-time)
Σήμερα έγραψα… = Today I wrote… (completed in the past)

The original sentence is neutral, so context decides the nuance.

What exactly is κοιμηθώ? Why not use κοιμάμαι after πριν?

κοιμηθώ is the aorist subjunctive (perfective) form of the verb κοιμάμαι (to sleep).

κοιμάμαι = present (imperfective) → I sleep / I am sleeping
να κοιμηθώ (or here just κοιμηθώ after πριν) = aorist subjunctive (perfective) → that I (should) go to sleep / sleep (once, as a single event)

After πριν (before), when it introduces a clause (πριν + verb), Greek normally uses the subjunctive, and for a single future event we choose the aorist:

πριν κοιμηθώ = before I (go to) sleep (one event in the future relative to “write”)

Using πριν κοιμάμαι here would be ungrammatical.

Why is there no να in πριν κοιμηθώ? I thought subjunctive usually needs να.

You’re right that the subjunctive is usually introduced by να (e.g. να κοιμηθώ).

However, several conjunctions in Greek take the subjunctive directly, without “να”. One of them is πριν when it means before + clause:

πριν κοιμηθώ = before I sleep
όταν φτάσω = when I arrive
αν πάω = if I go

So:

πριν κοιμηθώ is the normal, standard form.
πριν να κοιμηθώ is heard in some dialects/colloquial speech, but “πριν + να” is usually avoided in careful standard Greek.

Could I say πριν από τον ύπνο instead of πριν κοιμηθώ?

Yes, but the nuance changes slightly.

πριν κοιμηθώ = before I (go to) sleep → focuses on the action of you going to sleep.
πριν από τον ύπνο = before (the) sleep → uses a noun instead of a verb; it’s a bit more abstract or formal.

Your original sentence:

Σήμερα γράφω τις σκέψεις μου στο ημερολόγιό μου πριν κοιμηθώ.

If you change it:

Σήμερα γράφω τις σκέψεις μου στο ημερολόγιό μου πριν από τον ύπνο.

It’s still correct and understandable, just slightly more “nouny” and less direct than using the verb.

Can I change the word order? For example, can I say Σήμερα γράφω στο ημερολόγιό μου τις σκέψεις μου?

Yes. Greek word order is relatively flexible, especially for objects.

All of these are possible and natural:

Σήμερα γράφω τις σκέψεις μου στο ημερολόγιό μου.
Σήμερα γράφω στο ημερολόγιό μου τις σκέψεις μου.

The difference is subtle:

Putting τις σκέψεις μου earlier slightly emphasizes what you write.
Putting στο ημερολόγιό μου earlier slightly emphasizes where you write.

Both are fine. The original order is probably the most neutral.

Can I move σήμερα to another position in the sentence?

Yes. Common options include:

Σήμερα γράφω τις σκέψεις μου στο ημερολόγιό μου πριν κοιμηθώ.
Γράφω σήμερα τις σκέψεις μου στο ημερολόγιό μου πριν κοιμηθώ.

Both are acceptable and natural. Σήμερα often appears at the very beginning, but putting it after the verb is also fine. If you put it at the very end, it can sound a bit unusual in this particular sentence:

Γράφω τις σκέψεις μου στο ημερολόγιό μου πριν κοιμηθώ σήμερα.
(grammatical, but can sound like you are emphasizing “today, not on some other day” or even slightly ambiguous about whether “today” modifies the sleeping or the writing)

Why is ημερολόγιό in the accusative case after στο?

The preposition σε (which becomes στο, στη, στον, etc.) always takes a noun in the accusative case.

στο σπίτι (από το σπίτι)
στην πόλη (από την πόλη)
στο ημερολόγιό μου (από το ημερολόγιο)

So το ημερολόγιο (nominative) becomes το ημερολόγιό μου in the accusative after σε, and the article + noun are wrapped into the contraction στο ημερολόγιό μου.