Το βράδυ ζεσταίνω λίγο νερό στην κατσαρόλα.

In Greek, a time expression like το πρωί, το μεσημέρι, το βράδυ often works on its own as an adverbial of time and is translated as in the morning / at noon / in the evening.

Literally, το βράδυ is the evening / the night (neuter noun το βράδυ). But when used this way in a sentence, it means in the evening / at night without needing a separate preposition.

For habitual actions you’ll also hear:

• Τα βράδια ζεσταίνω λίγο νερό… = In the evenings I heat a bit of water… (regularly, on most evenings)

Greek usually drops subject pronouns because the verb ending already shows the person and number.

• ζεσταίνω = I warm / heat (1st person singular)
  So Εγώ ζεσταίνω is grammatically correct but normally unnecessary unless you want to emphasize I (as opposed to someone else):

• Εγώ ζεσταίνω λίγο νερό… = I (and not someone else) warm a bit of water…

ζεσταίνω means to warm, to heat (something).

In this sentence it is present tense, 1st person singular:

• (εγώ) ζεσταίνω = I warm / I am warming

Modern Greek present tense covers both English simple present and present continuous, so ζεσταίνω can mean:

• I warm (in general/habitually)
• I am warming (right now)

Context decides which feels more natural in English. Here, with το βράδυ, it usually sounds like a regular habit:
In the evening I warm up a bit of water…

ζεσταίνω is a regular -ω verb. Present tense:

• (εγώ) ζεσταίνω – I warm
• (εσύ) ζεσταίνεις – you warm (singular)
• (αυτός / αυτή / αυτό) ζεσταίνει – he / she / it warms
• (εμείς) ζεσταίνουμε – we warm
• (εσείς) ζεσταίνετε – you warm (plural / formal)
• (αυτοί / αυτές / αυτά) ζεσταίνουν(ε) – they warm

Spelling note: αι is pronounced like e (as in “bed”), so ζεσταίνω is pronounced roughly ze-STÉ-no.

Yes, mainly in style and typical use:

• ζεσταίνω is the everyday, colloquial verb:

  • ζεσταίνω νερό, φαγητό, το σπίτι = I heat water, food, the house.

• θερμαίνω is more formal / technical and more common in scientific or written language:

  • Η συσκευή θερμαίνει το νερό. = The device heats the water.

In normal speech about daily life, you’d almost always say ζεσταίνω.

Because λίγο νερό means a little (some) water, an indefinite quantity. In Greek, when you talk about an unspecified amount of a mass noun, you usually don’t use the article:

• λίγο νερό = a little / some water
• πολύ νερό = a lot of water
• νερό (by itself) can also mean “water” in general

If you say το νερό, you usually mean the water in a specific context:

• Ζεσταίνω το νερό. = I heat the water (the particular water we’ve mentioned).

In this sentence, we’re not talking about some specific known water, just “a bit of water”, so λίγο νερό is natural.

Here, λίγο is functioning like an adjective describing the amount of the noun νερό:

• λίγο νερό = a little water / a small amount of water

Compare:

• Πίνω λίγο νερό. = I drink a little water.

When λίγο modifies a verb, it works more like an adverb meaning a little / a bit:

• Κρυώνω λίγο. = I’m a bit cold.
• Πεινάω λίγο. = I’m a little hungry.

In your sentence, it’s quantity: how much water is being heated.

στην = σε + την contracted.

• σε = in, at, on, into (general preposition)
• την = the (feminine, singular, accusative)

So:

• σε την κατσαρόλα → στην κατσαρόλα = in the pot / in the saucepan

This is very common in Greek; σε + article almost always contracts:

• σε τον → στον
• σε την → στην
• σε το → στο
• σε τις → στις, etc.

κατσαρόλα (pot, saucepan) is a feminine noun:

• η κατσαρόλα – the pot (nominative)
• την κατσαρόλα – the pot (accusative)

After the preposition σε, Greek uses the accusative case, so:

• σε + την κατσαρόλα → στην κατσαρόλα

That’s why you see κατσαρόλα in this form.

Yes, Greek word order is fairly flexible, especially for adverbials like το βράδυ and λίγο.

Some natural options:

• Το βράδυ ζεσταίνω λίγο νερό στην κατσαρόλα. (original)
• Ζεσταίνω το βράδυ λίγο νερό στην κατσαρόλα.
• Το βράδυ ζεσταίνω στην κατσαρόλα λίγο νερό.

The basic structure (subject–verb–object) is preserved, but adverbs and time expressions can move around for emphasis or style. The original order is very typical and neutral.

Greek doesn’t use phrasal verbs in the same way English does. The single verb ζεσταίνω already contains the idea “make something warm / warmer”.

So:

• ζεσταίνω λίγο νερό = I warm / warm up a bit of water

You don’t need an extra particle like “up”. Other examples:

• ανοίγω την πόρτα = I open the door (not “open up”)
• καθαρίζω το δωμάτιο = I clean the room (not “clean up”)

In the right context, yes. Modern Greek present tense can sometimes refer to the near future, similar to English:

• Αύριο φεύγω. = I leave tomorrow / I’m leaving tomorrow.

With το βράδυ, if it’s clear you’re talking about tonight’s plan, it could be understood as future:

• Το βράδυ ζεσταίνω λίγο νερό στην κατσαρόλα και κάνω μπάνιο.
  = Tonight I’ll heat some water in the pot and take a bath.

However, without extra context, many listeners would first interpret it as a habitual action (what you usually do in the evenings).