A szósz, ami a lábasban van, meleg.

Because in Hungarian, the present-tense verb to be is usually omitted in the 3rd person singular when the predicate is a noun or an adjective.

So:

• A szósz meleg. = The sauce is warm.

not:

• A szósz meleg van.

English needs is, but Hungarian normally does not in this pattern.

Here van is needed because it expresses location/existence: the sauce is in the pot.

Compare:

• A szósz meleg. = The sauce is warm. → no van
• A szósz a lábasban van. = The sauce is in the pot. → van stays

So the sentence has both patterns at once:

• main clause: A szósz ... meleg
• relative clause: ami a lábasban van

Ami is a relative pronoun here, meaning that or which.

It introduces the part ami a lábasban van, which describes a szósz.

So structurally it is like:

• the sauce that/which is in the pot

In everyday Hungarian, ami is very common and natural for things.

Because ami is used for things, while aki is used for people.

• ami = that/which, referring to a thing
• aki = who, referring to a person

So:

• a szósz, ami... = the sauce that...
• a szakács, aki... = the cook who...

In Hungarian spelling, a relative clause introduced by words like aki, ami, amely, ahol, etc. is normally set off with a comma.

Since the relative clause is inserted in the middle of the sentence, you get:

• one comma before it
• one comma after it

So:

• A szósz, ami a lábasban van, meleg.

This is more regular in Hungarian than in English, where commas depend more on whether the clause is restrictive or non-restrictive.

Lábasban can be broken down like this:

• lábas = pot, saucepan
• -ban = in

So lábasban literally means in a/the pot.

This -ban/-ben ending is called the inessive case, and it means inside something.

Because of vowel harmony.

Hungarian suffixes often come in two versions:

• -ban
• -ben

The noun lábas contains back vowels, so it takes -ban:

• lábas + ban = lábasban

A word with front vowels would take -ben instead.

Because both nouns are definite:

• A szósz = the sauce
• a lábasban = in the pot

So the sentence talks about a specific sauce and a specific pot.

Also, in Hungarian the definite article still appears before a noun even when that noun has a case ending:

• a lábasban
• not just lábasban, if the pot is definite

Yes. A is the definite article, so a szósz means the sauce, not just a sauce in general.

If you wanted an indefinite meaning, you would use egy:

• egy szósz = a sauce

So the sentence is talking about a sauce already identifiable from context.

That is a very natural Hungarian word order here.

The sentence starts with the thing being talked about:

• A szósz

then adds extra information about which sauce:

• ami a lábasban van

and finally gives the main statement:

• meleg

Putting meleg at the end makes it sound like the key piece of information.

You can sometimes change word order for emphasis, but this version is very normal.

Meleg is an adjective, meaning warm.

In this sentence it is used predicatively, meaning it is the main description of the subject:

• A szósz meleg. = The sauce is warm.

A few useful points:

• it does not show gender, because Hungarian has no grammatical gender
• here it is in its basic singular form
• with a plural subject, the predicate can be plural too: A szószok melegek. = The sauces are warm.

Yes. A common alternative is:

• A lábasban lévő szósz meleg.

This means essentially the same thing: The sauce in the pot is warm.

Compared with ami a lábasban van:

• ami ... van sounds very common and conversational
• lévő sounds a bit more compact and somewhat more written/formal

Both are correct.

Two important Hungarian spelling rules show up here:

• sz = English s
• s = English sh

So:

• szósz has an s sound at both the beginning and end
• lábas ends with a sh sound

Very rough English-style approximations:

• szósz ≈ sohss
• lábas ≈ laa-bosh

Also:

• ó is a long vowel
• á is also long

So both words have a noticeably long first vowel.