Το πουλόβερ αυτό είναι ζεστό, αλλά θέλω άλλο που να μην κρατάει τόση ζέστη μέσα στο γραφείο.

Both are correct, but they are used a little differently.

• αυτό το πουλόβερ is the most neutral and very common way to say this sweater
• το πουλόβερ αυτό is also correct and can sound slightly more emphatic, a bit like this sweater here or this particular sweater

Greek often allows the demonstrative to come either:

• before the noun: αυτό το πουλόβερ
• after the noun: το πουλόβερ αυτό

Notice that when a demonstrative is used, Greek normally also keeps the article:

• αυτό το βιβλίο
• το βιβλίο αυτό

So το is not extra here; it is part of normal Greek structure.

Because πουλόβερ is a neuter singular noun, so it takes the neuter singular definite article το.

• το πουλόβερ = the sweater
• το πουλόβερ αυτό = this sweater

Even though English does not use the in this sweater, Greek usually does use the article together with demonstratives.

ζεστό means warm.

It is neuter singular because it agrees with πουλόβερ, which is also neuter singular:

• το πουλόβερ → neuter singular
• είναι ζεστό → is warm

Greek adjectives change form to match the gender, number, and case of the noun they describe.

For example:

• ο καφές είναι ζεστός = the coffee is hot/warm
• η σούπα είναι ζεστή = the soup is hot/warm
• το πουλόβερ είναι ζεστό = the sweater is warm

They are related, but they are different parts of speech.

• ζεστό = warm, an adjective
• ζέστη = heat/warmth, a noun

So:

• Το πουλόβερ αυτό είναι ζεστό = This sweater is warm
• κρατάει τόση ζέστη = it keeps/traps so much heat

A useful way to see it:

• adjective: warm
• noun: warmth / heat

Because Greek, like English, can leave out a noun when it is obvious from context.

Here, άλλο means another one or a different one, with πουλόβερ understood:

• θέλω άλλο = I want another one
• more fully: θέλω άλλο πουλόβερ = I want another sweater

Since πουλόβερ is neuter singular, άλλο is also neuter singular.

Here που is a relative word meaning something like that/which.

It introduces a clause describing άλλο:

• θέλω άλλο που... = I want another one that...

So:

• άλλο που να μην κρατάει... = another one that does not keep...

In everyday Greek, που is very commonly used where English might use that, which, or even omit the relative word entirely.

This is an important Greek pattern.

The speaker is not talking about a specific sweater that already exists and is known. They are describing the kind of sweater they want. Because of that, Greek often uses:

• που να + verb

This gives the idea of that would..., that should..., or that can..., depending on context.

So:

• θέλω άλλο που να μην κρατάει τόση ζέστη
  means something like
• I want another one that won’t / doesn’t / wouldn’t keep so much heat

If you said που δεν κρατάει, it would sound more like you already have a specific known sweater in mind, one that in fact does not keep so much heat.

So the να structure is used because the sweater is non-specific and desired, not identified.

Because after να, Greek uses μη(ν) for negation, not δεν.

Compare:

• κρατάει = it keeps
• δεν κρατάει = it does not keep
• να κρατάει = to keep / that it keep
• να μην κρατάει = to not keep / that it not keep

So:

• δεν is used with ordinary statements
• μην is used with να and in other non-indicative contexts

This is a very common rule in Greek.

Literally, κρατάει comes from κρατάω / κρατώ, which often means hold, keep, or retain.

In this sentence, it means something like:

• keep in
• retain
• trap

So να μην κρατάει τόση ζέστη means:

• not to keep so much heat
• not to trap so much warmth

For clothing, this is a very natural way to speak in Greek.

Also, both spellings/forms are common:

• κρατάει
• κρατά

They mean the same thing here. κρατάει is often a bit more common in everyday writing.

τόση means so much / that much / such a lot of, and it agrees with ζέστη, which is a feminine singular noun.

So:

• τόση ζέστη = so much heat

Because ζέστη is uncountable, English uses much, not many.

Agreement:

• τόσος for masculine nouns
• τόση for feminine nouns
• τόσο for neuter nouns

Examples:

• τόσος θόρυβος = so much noise
• τόση ζέστη = so much heat
• τόσο νερό = so much water

It means inside the office or, in natural English here, simply in the office.

Breakdown:

• μέσα = inside
• στο = σε + το = in the
• γραφείο = office

So:

• μέσα στο γραφείο = inside the office

The use of μέσα adds the feeling of being physically indoors, inside that space. In this sentence, it helps explain that the sweater is too warm specifically when the speaker is in the office.

It can mean either, depending on context.

• το γραφείο = desk
• το γραφείο = office

Here it clearly means office, because μέσα στο γραφείο means inside the office, not inside the desk.

Context usually makes the meaning obvious.

Not exactly. In this sentence, ζεστό is best understood as warm rather than hot.

For clothes, ζεστός / ζεστή / ζεστό often means:

• warm
• good for keeping you warm

So Το πουλόβερ αυτό είναι ζεστό means the sweater is warm, or it keeps the speaker too warm.

English might also say:

• This sweater is very warm
• This sweater is too warm

depending on the situation.

Because είναι belongs only to the first clause:

• Το πουλόβερ αυτό είναι ζεστό = This sweater is warm

The subject is το πουλόβερ αυτό, which is singular, so the verb is singular:

• είναι = is

The rest of the sentence is a new clause:

• αλλά θέλω άλλο... = but I want another one...

So each verb agrees with its own subject:

• είναι → subject: το πουλόβερ αυτό
• θέλω → subject: understood I
• κρατάει → subject: understood the other sweater

Yes, θέλω ένα άλλο is possible, but it is slightly different in feel.

• θέλω άλλο = I want another one / a different one
• θέλω ένα άλλο = I want another one, with a slightly more explicit one

In many contexts, Greek naturally drops ένα here. The shorter θέλω άλλο sounds very normal.

If you wanted to be fully explicit, you could also say:

• θέλω άλλο πουλόβερ
• θέλω ένα άλλο πουλόβερ

All are possible, but the original sentence is very natural.

Yes, very much so.

The sentence uses several very natural Greek patterns:

• Το πουλόβερ αυτό... αλλά θέλω άλλο
• που να μην... to describe the kind of thing wanted
• κρατάει ζέστη to talk about retaining warmth

A Greek speaker would understand it immediately as:

• this sweater is too warm for office use
• the speaker wants a different one that does not trap as much heat

So although the structure may feel different from English, it is idiomatic Greek.