Çorba kaynamaya başlayınca altını kıs.

Başlayınca means when it starts / once it starts.

It comes from the verb başlamak (to start, to begin) plus the ending -ınca / -ince / -unca / -ünce, which often means when.

So:

• başlamak = to start
• başlayınca = when it starts

In this sentence, it is part of kaynamaya başlayınca = when it starts to boil.

Because in Turkish, başlamak usually takes a verbal noun with the dative ending.

The pattern is:

• Verb stem + -ma/-me + -(y)a/-(y)e + başlamak

So:

• kaynamak = to boil
• kaynama = boiling / to boil as a verbal noun
• kaynamaya başlamak = to start boiling

This is a very common structure in Turkish:

• ağlamaya başladı = he/she started crying
• konuşmaya başladılar = they started talking

Kaynamaya is made of:

• kaynama = boiling / to boil
• -a = dative ending
• the extra y is a buffer consonant

Turkish often inserts y between two vowels to make pronunciation easier.

So it is:

• kaynama + a → kaynamaya

This is completely normal and very common in Turkish grammar.

Usually, yes: -ınca / -ince / -unca / -ünce often means when, once, or after in the sense of as soon as that happens.

Examples:

• eve gelince = when he/she comes home
• bitirince = when he/she finishes
• görünce = when he/she sees

So:

• kaynamaya başlayınca = when it starts to boil
• more naturally in English here: once the soup starts boiling

It often gives a sense of the moment that happens, do the next thing.

This is a very common cooking expression in Turkish.

Literally:

• alt = bottom, lower part, underneath
• altını kıs = reduce its underside / lower what is underneath it

But idiomatically, in cooking, it means:

• turn down the heat
• lower the flame under it

So when Turkish says çorbanın altını kıs or simply altını kıs, it refers to the heat under the pot.

This is one of those expressions that makes perfect sense in Turkish cooking language, even though the literal wording sounds odd in English.

Because altını is the object of the verb kıs.

You can think of it like this:

• altı = its underside / its bottom
• altını = its underside as a definite direct object

More technically:

• altı contains a 3rd person possessive idea: its underside
• -nı is the accusative ending used after possessed forms

So:

• altı = its bottom / the part under it
• altını kıs = lower that under-part / lower the heat under it

Even if you do not analyze every ending, it is worth learning altını kısmak as a set phrase meaning to turn down the heat under something.

Kıs is the informal singular imperative of kısmak.

• kısmak = to reduce, to turn down, to lower
• kıs! = reduce it! / turn it down!

In this sentence, it is giving an instruction:

• altını kıs = turn down the heat

Other related forms:

• kısın = turn it down! (plural or polite singular)
• kısıyorum = I am turning it down
• kıstı = he/she turned it down

It is understood, not said.

Turkish often drops subject pronouns when they are clear from the verb form or the situation. Since kıs is an imperative, the hidden subject is sen (you, singular informal).

So the full idea is:

• (Sen) çorba kaynamaya başlayınca altını kıs.

But saying sen here would usually be unnecessary.

Turkish does not have a word exactly like English the.

So çorba can mean:

• soup
• the soup

The exact meaning comes from context.

In this sentence, because you are clearly talking about a specific soup being cooked, English naturally translates it as the soup.

Also, çorba is the subject here, so it appears in its basic form with no extra case ending.

Yes. That would also be natural, but it is slightly different.

• Çorba kaynamaya başlayınca... = when the soup starts to boil
• Çorba kaynayınca... = when the soup boils / when it has come to a boil

The sentence you were given emphasizes the beginning of boiling. That can matter in cooking instructions, because it means as soon as it starts boiling, lower the heat.

So both are possible, but kaynamaya başlayınca is a bit more specific.

Because that is a very natural word order in Turkish.

Turkish often puts:

• time expressions
• condition clauses
• background information

before the main action.

So:

• Çorba kaynamaya başlayınca altını kıs. = When the soup starts to boil, turn down the heat.

This order sounds very normal and smooth in Turkish.

You could rearrange parts in some contexts, but the given version is the most natural kind of instruction.

Yes, it is informal because of kıs.

If you want to be more polite or address more than one person, you can say:

• Çorba kaynamaya başlayınca altını kısın.

That can mean:

• Turn down the heat when the soup starts to boil.
  addressed politely to one person, or
• addressed to multiple people

So:

• kıs = informal singular
• kısın = polite singular or plural

Yes, several very useful ones:

• çorba = soup
• kaynamak = to boil
• başlamak = to start, begin
• kısmak = to reduce, turn down
• alt = bottom, underneath

Especially useful chunks to remember are:

• -maya / -meye başlamak = to start doing something
• -ınca / -ince = when / once
• altını kısmak = to turn down the heat under something

If you learn those as patterns, you will understand many similar cooking and everyday sentences.