Tarhanayı sıcak suyla karıştırınca çorba hemen hazır oluyor.

Because tarhanayı has the accusative ending -yı.

In Turkish, a direct object often takes the accusative when it is specific/definite. Here, you are talking about the tarhana that you are mixing, not tarhana in a vague general sense.

• tarhana = tarhana
• tarhanayı = the tarhana / tarhana as a specific object

The -y- is a buffer consonant added because the noun ends in a vowel.

Suyla means with water.

It comes from:

• su = water
• -la / -le = with, by means of

So you would expect su + la, but Turkish inserts a buffer y between two vowels:

• su + yla → suyla

This is very common in Turkish when a suffix beginning with a vowel-related form is attached to a word ending in a vowel.

Karıştırınca means when (you) mix it, once (you) mix it, or upon mixing it.

It is built from:

• karıştır- = to mix, stir
• -ınca / -ince / -unca / -ünce = when, once, after doing

So:

• karıştırınca = when one mixes it / once you mix it

This form is called a converb or adverbial verb form. It connects one action to another:

• Tarhanayı sıcak suyla karıştırınca, çorba hemen hazır oluyor.
• When you mix tarhana with hot water, the soup is ready right away.

Turkish often leaves the subject unstated when it is clear from context or when the meaning is general.

Here, karıştırınca can be understood like:

• when you mix it
• when one mixes it
• when it is mixed

In everyday Turkish, this kind of sentence often has a general instruction-like meaning, similar to English recipe language.

So even though no pronoun is written, the sentence is perfectly natural.

Both are possible in different contexts, but hazır oluyor gives the idea of becoming ready or ending up ready.

• hazır = ready
• hazır olmak = to be ready / become ready
• hazır oluyor = becomes ready / is getting ready / ends up ready

In this sentence, oluyor sounds natural because it describes the result of the mixing.

So the meaning is not just the soup is ready, but more like:

• the soup becomes ready immediately
• the soup is ready right away

In Turkish, -yor is often used not only for actions happening right now, but also for:

• general present situations
• habitual statements
• descriptions of how something works

So hazır oluyor here can mean something like:

• it gets ready
• it comes out ready
• it is ready

If you said hazır olur, that could also be possible in some contexts, but hazır oluyor sounds very natural in everyday explanatory speech.

Hemen means immediately, right away, or at once.

It adds the idea that the soup does not take long after mixing:

• hemen hazır oluyor = it becomes ready immediately

It is a very common adverb in Turkish.

Because çorba is the subject of the main clause.

In the sentence:

• Tarhanayı sıcak suyla karıştırınca = when you mix the tarhana with hot water
• çorba hemen hazır oluyor = the soup becomes ready immediately

Here, çorba is doing the action of becoming ready, so it stays in the basic form with no case ending.

This is a very useful distinction:

• karıştırmak = to mix, stir, confuse something
• karışmak = to become mixed, get involved, be confused

So:

• Tarhanayı karıştırmak = to mix/stir the tarhana
• Tarhana karıştı = the tarhana got mixed

In your sentence, karıştırınca is used because someone is actively mixing it.

Not completely. Turkish word order is flexible, but this sentence has a very natural structure.

Current order:

• Tarhanayı sıcak suyla karıştırınca = when you mix tarhana with hot water
• çorba hemen hazır oluyor = the soup becomes ready immediately

A few points:

• The main verb usually comes at the end
• The part before it can move around for emphasis
• The converb clause often comes before the main clause

So this sentence sounds normal and neutral. Turkish allows variation, but this order is a very standard way to say it.

The suffix -ınca links the first action directly to the second and gives a meaning like:

• when
• once
• as soon as

So it is stronger and more connected than just listing two actions separately.

Compare the idea:

• Tarhanayı sıcak suyla karıştırırsın. Çorba hazır olur.
  = You mix the tarhana with hot water. The soup is ready.

• Tarhanayı sıcak suyla karıştırınca çorba hemen hazır oluyor.
  = When/once you mix the tarhana with hot water, the soup is ready immediately.

The second version feels smoother and more natural for describing a process.

Yes. Sıcak suyla literally means with hot water, and placing it before the verb is completely normal in Turkish.

Breakdown:

• sıcak = hot
• su = water
• suyla = with water

So:

• sıcak suyla = with hot water

Turkish commonly puts instruments or means before the verb:

• kaşıkla karıştırmak = to stir with a spoon
• elle yapmak = to do by hand
• sıcak suyla karıştırmak = to mix with hot water

So this phrasing is very natural.