Learn Hardcore Greek: Μερικές φορές σκέφτομαι το παρελθόν όταν χαλαρώνω στο σαλόνι. - Sometimes I think about the past when I relax in the living room.

Μερικές φορές literally means some times or a few times, and together it works as the adverb sometimes.
μερικές is the feminine plural form of μερικός / μερική / μερικό (some), because it agrees in gender and number with φορές, the plural of η φορά (time, occasion).
So grammatically it is some (fem. pl.) + times (fem. pl.), but in practice you just learn it as the fixed expression μερικές φορές = sometimes.

Can I move Μερικές φορές to another position in the sentence?

Yes. Adverbials of time and frequency are quite flexible in Greek. For example, you can also say:

Σκέφτομαι μερικές φορές το παρελθόν όταν χαλαρώνω στο σαλόνι.
Όταν χαλαρώνω στο σαλόνι, μερικές φορές σκέφτομαι το παρελθόν.

Putting Μερικές φορές at the beginning slightly emphasizes the idea of sometimes, but all these options are natural.

Why is the verb σκέφτομαι in that form, and where is the subject I?

σκέφτομαι is what’s called a deponent verb in Greek: it uses middle/passive endings but has an active meaning (I think). There is no active form σκέφτω in Modern Greek; the basic form you learn is σκέφτομαι.

The subject I is contained in the verb ending -ομαι, which marks 1st person singular. Greek usually drops subject pronouns, so you normally say just σκέφτομαι rather than εγώ σκέφτομαι, unless you want to emphasize I (as opposed to someone else).

Why don’t we say σκέφτομαι για το παρελθόν here? In English we say think about the past.

In Greek, σκέφτομαι normally takes a direct object without a preposition: σκέφτομαι κάτι = I think about something / I think of something.
So σκέφτομαι το παρελθόν already means I think about the past; για is not required.

You can also hear σκέφτομαι για το παρελθόν, but that often feels a bit more colloquial or can suggest “I am considering / reflecting on the past” as a topic. The most neutral, standard form here is σκέφτομαι το παρελθόν.

The verbs are in the present tense. How does that match the meaning with sometimes?

In Greek, the present tense covers both simple present and present continuous meanings.
With μερικές φορές, the present here expresses a habitual action:

σκέφτομαι = I (sometimes) think / I tend to think
χαλαρώνω = I (when I) relax / whenever I relax

So the sentence means something like I sometimes think about the past when I relax in the living room, describing a regular pattern, not necessarily something happening right now.

Why is it το παρελθόν with a definite article, and what case is it?

το παρελθόν is in the accusative case, because it is the direct object of σκέφτομαι. In neuter nouns, nominative and accusative singular look the same, so το παρελθόν could be either subject or object depending on context; here it is the object.

Greek uses the definite article with abstract nouns more freely than English. Here, as in English the past, the article το feels natural and is normally included: you would usually say σκέφτομαι το παρελθόν, not bare σκέφτομαι παρελθόν.

Why do we use όταν χαλαρώνω and not όταν χαλαρώσω?

όταν + present (χαλαρώνω) is used for repeated or habitual situations:
Όταν χαλαρώνω στο σαλόνι… = When(ever) I relax in the living room… (it happens from time to time).

όταν + subjunctive (χαλαρώσω) is used for a single, usually future event:
Όταν χαλαρώσω μετά τη δουλειά, θα σου τηλεφωνήσω. = When I (have) relaxed after work, I’ll call you.

In your sentence we are talking about what generally happens whenever you relax there, so όταν χαλαρώνω is the correct choice.

What exactly does στο mean in στο σαλόνι?

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

σε + το → στο

Greek almost always contracts σε with a following definite article:

σε + τον → στον
σε + τη(ν) → στη(ν)
σε + τους → στους, etc.

In this sentence, στο σαλόνι means in the living room (a location).

What is the gender and form of σαλόνι, and why doesn’t it change after στο?

σαλόνι is a neuter noun. In the singular its main forms are:

το σαλόνι (nominative / accusative)
του σαλονιού (genitive)

The preposition σε always governs the accusative, so grammatically it is σε + το σαλόνι (accusative). But for neuter nouns, nominative and accusative have the same form, so you see το σαλόνι in both roles.

A common synonym you may meet is το καθιστικό (the sitting room / living room), which can be used similarly: στο καθιστικό.

How do you pronounce the whole sentence, and what sounds are tricky for English speakers?

A rough stressed-syllable guide (caps show the stressed syllable):

Me-ri-KÉS fo-RÉS SKÉF-to-me to pa-rel-THÓN Ó-tan ha-la-RÓ-no sto sa-LÓ-ni.

Key sounds:

χ in χαλαρώνω is like a hard h, similar to German ch in Bach.
θ in παρελθόν is like th in think, not like this.
ρ is a tapped or slightly rolled r.
αι (in -μαι of σκέφτομαι) is pronounced like e in met; οι (in σαλόνι) is like ee in see.
Each word has only one stressed syllable, marked in writing by the accent (´) over a vowel in Greek.