Όταν περπατάω γρήγορα το καλοκαίρι, ζεσταίνομαι πολύ.

Greek often leaves out subject pronouns because the verb ending already shows the person and number.

• περπατάω = I walk / I am walking
• ζεσταίνομαι = I get hot / I feel hot

So Greek does not need εγώ unless the speaker wants extra emphasis, contrast, or clarity.

For example:

• Όταν περπατάω γρήγορα, ζεσταίνομαι πολύ. = normal
• Όταν εγώ περπατάω γρήγορα, ζεσταίνομαι πολύ. = more emphatic, like when I walk fast, I get very hot

Όταν means when.

In this sentence, it introduces a time clause:

• Όταν περπατάω γρήγορα το καλοκαίρι = When I walk fast in the summer

A very common Greek pattern is:

• Όταν + present verb, present verb

This often expresses something habitual or generally true:

• Όταν τρώω πολύ, νυστάζω. = When I eat a lot, I get sleepy.

So here it means a repeated/general situation, not just one single event.

Because the sentence describes a general, repeated situation.

• περπατάω = I walk / I am walking
• ζεσταίνομαι = I get hot / I feel hot

In English, we also often use the present for general truths:

• When I walk fast in the summer, I get very hot.

Greek does the same. It is not necessarily happening right now; it is something that typically happens.

They are two common forms of the same verb: to walk.

• περπατάω
• περπατώ

Both are correct in Modern Greek. περπατάω often sounds a bit more everyday and conversational, while περπατώ can sound slightly more concise or sometimes a bit more formal, depending on context and speaker. In real life, both are very common.

So:

• Όταν περπατάω γρήγορα...
• Όταν περπατώ γρήγορα...

Both are fine.

Here γρήγορα is an adverb, meaning quickly / fast.

It describes how you walk:

• περπατάω γρήγορα = I walk quickly / fast

In Greek, many adverbs have the same form as the neuter singular of the adjective:

• adjective: γρήγορος = fast
• feminine: γρήγορη
• neuter: γρήγορο
• adverb: γρήγορα

So even though it is related to the adjective, in this sentence it functions as an adverb.

Greek very often uses the definite article with time expressions, seasons, days, and similar nouns where English might not.

So:

• το καλοκαίρι = in the summer / during the summer

This is completely normal Greek.

Other similar examples:

• τον χειμώνα = in winter
• την Κυριακή = on Sunday
• το πρωί = in the morning

English and Greek simply use articles differently here.

In this sentence, it most naturally means in the summer / during summer, as a general time period.

So the sentence expresses a usual fact:

• When I walk fast in the summer, I get very hot.

If Greek wanted to make this summer especially clear, the context would usually help, or the speaker might say something more specific.

By itself, το καλοκαίρι often has a general seasonal meaning.

ζεσταίνομαι means something like:

• I get hot
• I warm up
• I feel hot

In this sentence, the best natural sense is I get very hot.

It comes from a verb that has a middle/passive-looking form, but here it is not a true passive in the English sense. It describes a change in your own state.

Compare:

• ζεσταίνω το φαγητό = I heat/warm the food
• ζεσταίνομαι = I get warm / I feel hot

So the ending does not mean I am heated by someone in a literal passive way here. It is just the normal verb form for this meaning.

It belongs to a group of Greek verbs often called middle/passive in form.

The important point for a learner is this:

• Greek has active verb forms, like ζεσταίνω
• and middle/passive forms, like ζεσταίνομαι

But the -μαι ending does not always mean a true passive translation in English. Sometimes it describes:

• a state
• a change happening to the subject
• something reflexive-like
• something naturally expressed that way in Greek

So here:

• ζεσταίνομαι πολύ = I get very hot

It is best learned as the normal form of the verb for this meaning, not as a literal passive construction.

Here πολύ is an adverb meaning very / a lot.

It modifies the verb ζεσταίνομαι:

• ζεσταίνομαι πολύ = I get very hot / I get hot a lot

In this sentence, the best translation is very.

Greek πολύ can function in different ways:

• as an adverb: τρέχω πολύ = I run a lot
• as an adverb of degree: είναι πολύ καλός = he is very good
• as an adjective form in other structures, depending on gender/case/number

Here it stays as πολύ because it is working adverbially.

The comma separates the when-clause from the main clause:

• Όταν περπατάω γρήγορα το καλοκαίρι, = subordinate clause
• ζεσταίνομαι πολύ. = main clause

Greek normally uses a comma in this kind of structure, especially when the time clause comes first.

So the punctuation works very much like English:

• When I walk fast in the summer, I get very hot.

Yes. Greek word order is more flexible than English, though not completely free. This sentence can be rearranged for emphasis or style.

For example:

• Όταν περπατάω γρήγορα το καλοκαίρι, ζεσταίνομαι πολύ. = neutral
• Το καλοκαίρι, όταν περπατάω γρήγορα, ζεσταίνομαι πολύ. = puts more focus on in the summer
• Ζεσταίνομαι πολύ όταν περπατάω γρήγορα το καλοκαίρι. = starts with the result

The original version is very natural and straightforward.

Yes. The Greek present tense can cover both ideas, depending on context:

• I walk fast = habitual
• I am walking fast = ongoing

In this sentence, because of Όταν and the general idea of το καλοκαίρι, the habitual meaning is the natural one:

• When I walk fast in the summer, I get very hot.

But the form περπατάω itself can also describe something happening right now in another context.

It is most naturally about a repeated or general situation.

The clues are:

• Όταν = when
• both verbs in the present
• το καλοκαίρι as a general seasonal time expression

So it means something like:

• Whenever I walk fast in the summer, I get very hot.

If a speaker wanted to talk about one specific event, Greek would usually make that clearer through context or different tense/aspect choices.