Learn Hardcore Greek: Μερικές φορές παίρνω ταξί από το λιμάνι μέχρι την πλατεία όταν βρέχει πολύ. - Sometimes I take a taxi from the port to the square when it rains a lot.

Μερικές φορές = sometimes / a few times (now and then)
Neutral, very common, good default for sometimes.
Κάποιες φορές = some times
Very close to μερικές φορές, but can sound a bit more vague or indefinite, like on some occasions.
Πολλές φορές = many times / often
This is closer to often than to sometimes; it implies it happens quite frequently.

So Μερικές φορές παίρνω ταξί… is the natural way to say Sometimes I take a taxi….

Why is there no εγώ before παίρνω? How do we know it means I?

Modern Greek usually drops subject pronouns, because the verb ending already shows the person:

παίρνω = I take
παίρνεις = you (sg) take
παίρνει = he/she/it takes
παίρνουμε = we take, etc.

So (εγώ) παίρνω ταξί clearly means I take a taxi even without εγώ.
You can say Εγώ παίρνω ταξί to emphasize I (as opposed to someone else):
Εγώ παίρνω ταξί, αλλά η φίλη μου πάει με τα πόδια.

Why is it παίρνω ταξί and not παίρνω ένα ταξί or παίρνω το ταξί?

All three are possible, but they’re used differently:

παίρνω ταξί
General, uncountable-like usage. Means I travel by taxi / I take a taxi in general, without focusing on “one specific taxi”. This is the most natural here.
παίρνω ένα ταξί
Literally I take a taxi (one taxi). Also correct; it sounds a bit more like choosing or grabbing a single cab at a specific moment.
παίρνω το ταξί
I take the taxi, referring to a specific taxi already known from context (e.g. the taxi we talked about). Less likely in a generic habit sentence.

In habits or general statements, Greek often prefers the bare noun:

Πηγαίνω σχολείο με λεωφορείο. = I go to school by bus.
Πάω στη δουλειά με τρένο. = I go to work by train.
Παίρνω ταξί… = I take a taxi / I go by taxi…

What’s the difference between από and μέχρι here? Why από το λιμάνι μέχρι την πλατεία?

Both are prepositions marking the two ends of movement or distance:

από = from
μέχρι = up to / until / as far as

So:

από το λιμάνι = from the port
μέχρι την πλατεία = up to the square / to the square

Together, από … μέχρι … means from … to …, just like in English.
You can also hear:

από το λιμάνι ως την πλατεία
από το λιμάνι μέχρι την πλατεία (as in the sentence)

μέχρι and ως are near-synonyms in this context; μέχρι is very common in everyday speech.

Why is it το λιμάνι but την πλατεία? Why do the articles change?

Greek nouns have gender, and the definite article agrees with the gender, number, and case.

το λιμάνι
- λιμάνι is neuter.
- το is the neuter singular article (in both nominative and accusative).
- After από (from), the noun is in the accusative, but for neuter, nominative and accusative look the same: το λιμάνι.
την πλατεία
- πλατεία is feminine.
- Feminine nominative singular: η πλατεία
- Feminine accusative singular: την πλατεία
- After μέχρι (to), the noun is also in the accusative, so we use την πλατεία.

So you see:

από το λιμάνι (neuter accusative)
μέχρι την πλατεία (feminine accusative)

Why is the verb in the present tense (παίρνω, βρέχει) if this is a general habit, not something happening right now?

Greek uses the simple present tense, Ενεστώτας, for:

Actions happening right now:
- Τώρα παίρνω ταξί. = I am taking a taxi now.
Regular or habitual actions (like English I take / I usually take):
- Μερικές φορές παίρνω ταξί… = Sometimes I (usually) take a taxi…
General truths:
- Στην Ελλάδα βρέχει το χειμώνα. = In Greece it rains in the winter.

So here παίρνω and βρέχει describe something that happens regularly when it rains a lot. This matches English I take a taxi (simple present for habits) and when it rains a lot (simple present for a repeated condition).

Why is it όταν βρέχει πολύ and not όταν πολύ βρέχει? Where does πολύ go?

In Greek, the adverb πολύ (meaning a lot / very much) typically comes after the verb it modifies:

βρέχει πολύ = it rains a lot
δουλεύω πολύ = I work a lot
σ’ αγαπώ πολύ = I love you very much

So:

όταν βρέχει πολύ = when it rains a lot

όταν πολύ βρέχει is grammatically possible but sounds unusual and stylistically marked; in everyday speech, you would naturally say όταν βρέχει πολύ.

What exactly does πολύ mean here? Could we say όταν έχει πολλή βροχή instead?

Here πολύ is an adverb modifying the verb βρέχει:

βρέχει πολύ = it rains a lot / it’s raining heavily

You could say όταν έχει πολλή βροχή (literally: when there is a lot of rain), but:

όταν βρέχει πολύ is shorter, more natural, and more common.
πολύ (adverb) = a lot, very much (modifies verbs, adjectives, other adverbs).
πολλή (adjective, feminine) = a lot of / much (modifies a feminine noun like βροχή: πολλή βροχή = a lot of rain).

So:

βρέχει πολύ (adverb)
έχει πολλή βροχή (adjective + noun)

In this sentence, the adverbial form is preferred.

Why is Μερικές φορές feminine plural? What is feminine here?

Μερικές φορές literally means “a few times”:

φορά = one time / one occasion (feminine noun)
φορές = times / occasions (feminine plural)

So:

μερικές is the feminine plural form of the adjective μερικός / μερική / μερικό (some / several / a few).
It agrees in gender and number with φορές (feminine plural).

Even though in English we say sometimes (one word, no obvious gender), in Greek the phrase is grammatically some times (fem. pl.):

μερικές φορές = literally some times → idiomatically sometimes

Could I change the word order to Μερικές φορές από το λιμάνι παίρνω ταξί or παίρνω από το λιμάνι ταξί?

Yes, Greek word order is relatively flexible, but there are preferences:

Μερικές φορές παίρνω ταξί από το λιμάνι μέχρι την πλατεία
Most neutral, natural order:
[frequency] – [verb + object] – [origin] – [destination].
Μερικές φορές από το λιμάνι παίρνω ταξί μέχρι την πλατεία
Correct, but από το λιμάνι is emphasized (from the port, not from somewhere else).
Μερικές φορές παίρνω από το λιμάνι ταξί…
Also possible; here you slightly highlight from the port as the source of the taxi, but it sounds less smooth than the original.

For everyday, unmarked speech, the original Μερικές φορές παίρνω ταξί από το λιμάνι μέχρι την πλατεία is the best choice.

Why is it όταν βρέχει and not αν βρέχει? Don’t both mean when / if?

Both όταν and αν can relate to conditions, but they differ:

όταν = when / whenever (for repeated or time-based events)
- Όταν βρέχει, παίρνω ταξί. = Whenever it rains, I take a taxi.
αν = if (for a condition that may or may not happen)
- Αν βρέχει, θα πάρω ταξί. = If it rains, I will take a taxi.

In your sentence we are talking about a habitual pattern: Whenever it rains a lot, I take a taxi.
So όταν βρέχει πολύ is the natural choice.

Do από and μέχρι always take the accusative case, like το λιμάνι and την πλατεία here?

With places and movement, από and μέχρι/ως are followed by the accusative:

από το σπίτι = from the house
από την πόλη = from the city
από το λιμάνι = from the port
μέχρι το σχολείο = up to / to the school
μέχρι την πλατεία = up to / to the square

So yes, in constructions like από X μέχρι Y (from X to Y), you will almost always see the nouns in the accusative case, just like in the sentence.

How do you pronounce λιμάνι, πλατεία, and μερικές φορές? Where is the stress?

Stress in Greek is marked by the accent (´) on a vowel.

λιμάνι → li-MA-ni
- Stress on -μά-: λι–ΜΑ–νι.
πλατεία → pla-TEE-a (3 “syllables” in Greek phonology: πλα–ΤΕΙ–α)
- Stress on -τε-: πλα–ΤΕΙ–α.
- The ει is pronounced like i in machine.
μερικές φορές → me-ri-KES fo-RES
- μερικές: με–ρι–ΚΕΣ (stress on the last syllable)
- φορές: φο–ΡΕΣ (stress on the last syllable)

Correct stress is important in Greek because moving the stress often changes the word or makes it hard to understand.