Learn Hardcore Greek: Η φίλη μου προετοιμάζει την εκδρομή και σημειώνει όλα τα βήματα στο ημερολόγιό της. - My friend female prepares the excursion and notes all the steps in her diary.

Η here is the definite article for feminine nouns in the nominative singular (the subject form): η φίλη = the (female) friend.
Because it’s at the start of the sentence, it’s capitalized (Η instead of η).
So Η φίλη = The (female) friend, just like Ο φίλος = The (male) friend.

Why is it Η φίλη μου and not μου φίλη?

Greek normally puts the unstressed possessive pronoun (my, your, his, etc.) after the noun:

η φίλη μου = my friend (literally: the friend my)

Putting it before (μου φίλη) is not standard modern Greek; it only appears in some fixed expressions or very old‑fashioned / poetic language.
So for everyday speech, use noun + μου/σου/του/της/μας/σας/τους:
το σπίτι μου, το βιβλίο σου, η φίλη μου.

What is the difference between μου and της in this sentence?

Both μου and της are genitive pronouns (possessive forms), but:

μου = my
της = her (or of her)

In η φίλη μου, μου tells us the friend belongs to the speaker: my friend.
In στο ημερολόγιό της, της tells us the diary belongs to some female person (usually the same friend, unless context says otherwise): in her diary.
Both are unstressed (enclitic) forms, so they attach closely to the word before them.

How do I know that προετοιμάζει and σημειώνει are third person singular?

Modern Greek present-tense verb endings are regular. For most -ω verbs:

(αυτός/αυτή/αυτό) προετοιμάζει = he/she/it prepares
(αυτός/αυτή/αυτό) σημειώνει = he/she/it notes down

The -ει or -ει / -εί / -ώνει type ending is typical for 3rd person singular in many verb groups.
Here, the subject Η φίλη μου is she, so:

η φίλη μου προετοιμάζει = my (female) friend prepares
η φίλη μου σημειώνει = my (female) friend notes down

Does the Greek present tense here mean “prepares” or “is preparing”?

Greek present tense usually covers both English simple present and present continuous.

So προετοιμάζει can be understood as:

she prepares the trip
or
she is preparing the trip

Context tells you whether it feels like a general habit or something happening now.
In this specific sentence, English speakers will usually choose “is preparing” and “is noting down”.

Why is it την εκδρομή and not just εκδρομή?

Greek uses the definite article much more often than English.
η εκδρομή = the trip, so in the accusative (direct object) it becomes την εκδρομή.

In Greek, you normally say:

προετοιμάζει την εκδρομή = she is preparing the trip

Dropping the article (προετοιμάζει εκδρομή) sounds incomplete or stylized; it’s not the normal way to say this.

Why is εκδρομή feminine and how does that affect the article?

The noun εκδρομή (trip / excursion) is grammatically feminine.
That means:

Nominative singular: η εκδρομή (the trip – as subject)
Accusative singular: την εκδρομή (the trip – as object)

The article η/την must agree in gender, number and case with the noun.
So you can’t say το εκδρομή; you must use η / την with εκδρομή.

Why is όλα τα βήματα structured like that? Why do we have both όλα and τα?

In Greek, “all the X” is normally:

όλα τα βήματα = all the steps

Structure:

όλα = all (neuter plural, agreeing with βήματα)
τα = the (neuter plural definite article)
βήματα = steps (neuter plural noun)

So όλα is a quantifier (all), and τα is the article (the).
This is a very standard pattern: όλα τα παιδιά, όλες τις μέρες, όλους τους φίλους, etc.

What does σημειώνει mean exactly? Is it just “writes”?

σημειώνει (from σημειώνω) means to note, jot down, record, mark down.
It often implies organized / purposeful writing, not just any writing.

Compare:

γράφει = he/she writes (more general)
σημειώνει = he/she notes down / makes notes of something

In this sentence, σημειώνει όλα τα βήματα suggests she is systematically writing down each step, like a checklist.

What is στο in στο ημερολόγιό της?

στο is the contraction of the preposition σε (in, at, on, to) plus the neuter article το (the):

σε + το = στο

So:

στο ημερολόγιό της = in her diary / into her diary

You’ll also see:

σε + τον = στον (to the / in the, masculine)
σε + την = στην (to the / in the, feminine)

Why does ημερολόγιό have an accent on the last ό in στο ημερολόγιό της?

The basic word is ημερολόγιο (diary), with the stress on -λό-:
η-με-ρο-ΛΟ-γιο (one accent).

When you add an enclitic pronoun after it (της, μου, etc.), Greek stress rules allow the accent to move so that the combined phrase doesn’t break the “no more than three syllables from the end” rule.
In practice, this results in adding a second accent:

το ημερολόγιό της (two accents: -λόγι- and -λό-γιό-)

This double accent is normal and correct when a word that’s already stressed on the third-from-last syllable is followed by an enclitic.

Could της in στο ημερολόγιό της refer to someone other than η φίλη μου?

Yes, in principle της just means her / of her, so it could refer to any female person mentioned in the broader context.
However, if there is no other female referent in the context, Greek speakers will naturally assume της refers back to η φίλη μου.

So by default:

Η φίλη μου … σημειώνει … στο ημερολόγιό της
is understood as
My friend … writes … in her (own) diary.

What is the difference between προετοιμάζει and ετοιμάζει?

Both are related to preparing, but:

ετοιμάζει (from ετοιμάζω) = prepares, gets something ready (general).
προετοιμάζει (from προετοιμάζω) = prepares in advance, often with more planning / organization implied.

In many everyday contexts they can overlap, but προετοιμάζει την εκδρομή sounds a bit more like she is systematically planning and organizing the trip, not just quickly getting it ready.