Ιδρώνω πολύ όταν έχει ζέστη στο λεωφορείο.

Greek often leaves out subject pronouns because the verb ending already shows who is doing the action.
Ιδρώνω means I sweat, so εγώ is not needed.

You could add εγώ for emphasis or contrast:

Εγώ ιδρώνω πολύ, αλλά αυτός όχι.
I sweat a lot, but he doesn’t.

Ιδρώνω is the 1st person singular present tense of the verb ιδρώνω = to sweat.

So:

• ιδρώνω = I sweat / I am sweating
• ιδρώνεις = you sweat
• ιδρώνει = he/she/it sweats

In this sentence, the present tense expresses a general/habitual situation: whenever the bus is hot, I sweat a lot.

Here it means a lot, because it is modifying a verb.

• Ιδρώνω πολύ = I sweat a lot

Greek πολύ can mean either very or a lot, depending on what it modifies:

• with a verb: a lot
• with an adjective/adverb: very

For example:

• Τρέχω πολύ. = I run a lot.
• Είναι πολύ ζεστό. = It is very hot.

That is the normal position here. In Greek, adverbs like πολύ often come after the verb:

• Ιδρώνω πολύ
• Δουλεύω πολύ
• Τρώω πολύ

Greek word order is more flexible than English, but Ιδρώνω πολύ is the most natural neutral order.

Όταν means when or whenever.

In this sentence, because the speaker is talking about something that happens repeatedly, όταν is best understood as whenever:

Ιδρώνω πολύ όταν έχει ζέστη στο λεωφορείο.
= I sweat a lot whenever it’s hot on the bus.

So even though English may translate it as when, the sense is habitual.

Because Greek uses the idiomatic expression έχει ζέστη to mean it is hot or there is heat.

So:

• έχει ζέστη = it’s hot
• έχει κρύο = it’s cold

You do not say είναι ζέστη, because ζέστη is a noun meaning heat, not an adjective.

If you want to use είναι, you need an adjective such as ζεστός / ζεστή / ζεστό:

• Το λεωφορείο είναι ζεστό. = The bus is warm/hot.

It is a noun.
Η ζέστη means the heat.

That is why έχει ζέστη literally looks like it has heat or there is heat, though idiomatically it means it’s hot.

Compare:

• ζέστη = heat (noun)
• ζεστός / ζεστή / ζεστό = warm / hot (adjective)

So:

• Έχει ζέστη. = It’s hot.
• Το νερό είναι ζεστό. = The water is warm.

This is an impersonal expression. Greek often uses 3rd person singular in weather, conditions, and general-state expressions.

So έχει ζέστη works like English it is hot, where it does not refer to a real thing.

Other similar Greek expressions are:

• Έχει κρύο. = It’s cold.
• Έχει φασαρία. = It’s noisy / There’s noise.

Because in this fixed expression, Greek normally says έχει ζέστη, not έχει τη ζέστη.

The noun is being used in a general, idiomatic way, just like in many weather or condition expressions.

So the normal pattern is:

• έχει ζέστη
• έχει κρύο
• έχει ησυχία
• έχει φασαρία

Adding the article would usually change the meaning or sound unnatural here.

Στο is the contraction of σε + το.

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

So:

• σε το λεωφορείο becomes στο λεωφορείο

This contraction is standard in modern Greek.

Other examples:

• σε + την = στην
• σε + τον = στον

Because λεωφορείο is a neuter noun, and in the singular its nominative and accusative forms are the same.

The preposition σε takes the accusative, but for this noun the form does not change:

• nominative: το λεωφορείο
• accusative: το λεωφορείο

So after στο, it still stays λεωφορείο.

In Greek, στο λεωφορείο is the normal way to express what English usually says as on the bus.

Greek uses σε for many location meanings that English divides into in, on, at, and to.

So although it may look like in the bus, the natural English translation is usually:

• στο λεωφορείο = on the bus

Yes. Greek word order is fairly flexible.

The given sentence is a natural neutral order:

Ιδρώνω πολύ όταν έχει ζέστη στο λεωφορείο.

But you could also say:

Όταν έχει ζέστη στο λεωφορείο, ιδρώνω πολύ.

This puts more focus on the when-clause first. The meaning stays basically the same.

You can also say κάνει ζέστη, and it is very common.

Both are possible:

• όταν έχει ζέστη στο λεωφορείο
• όταν κάνει ζέστη στο λεωφορείο

Very roughly:

• κάνει ζέστη is extremely common for general weather
• έχει ζέστη is also natural, especially when talking about the conditions in a particular place

So in this sentence, έχει ζέστη στο λεωφορείο sounds perfectly normal.