Bu kaçıncı ders?

• Bu = this
• kaçıncı = which one in order / what number
• ders = lesson, class

So the sentence literally means something like This, which-number lesson? In natural English, that becomes Which lesson is this? or What number lesson is this?

Kaçıncı asks about position in a sequence: first, second, third, and so on.

It comes from:

• kaç = how many
• the ordinal suffix -(I)ncI = a suffix that makes ordinal forms

So:

• kaç = how many
• kaçıncı = which number in order / the how-many-th one

The suffix changes form because of vowel harmony, so here it appears as -ıncı.

In Turkish, the verb to be is often left out in the present tense, especially in 3rd person sentences.

So:

• Bu kaçıncı ders? literally has no separate word for is
• but it means Which lesson is this?

This is completely normal Turkish.

A more formal or emphatic version could be:

• Bu kaçıncı derstir?

But in everyday speech, Bu kaçıncı ders? is the natural form.

This is a very important distinction:

• kaç asks about quantity: how many
• kaçıncı asks about order/rank: which one in sequence

Compare:

• Kaç ders var? = How many lessons are there?
• Bu kaçıncı ders? = Which lesson is this? / What number lesson is this?

So kaç is about number of items, while kaçıncı is about place in a series.

Turkish often puts the topic first, and here bu points to the thing being talked about: this lesson.

So the structure is roughly:

• Bu = this
• kaçıncı ders = what-number lesson

It is the most neutral and standard way to ask the question.

Turkish word order is somewhat flexible, but Bu kaçıncı ders? is the clearest default form.

Yes. Kaçıncı ders bu? is also possible.

The meaning stays the same, but it can sound a bit more:

• conversational
• expressive
• focused on the question word

So:

• Bu kaçıncı ders? = neutral, standard
• Kaçıncı ders bu? = also natural, with a slightly different emphasis

Both are used.

Because the sentence is talking about one specific lesson and asking its position in a sequence.

So:

• ders = lesson singular

Even in English, you also say:

• Which lesson is this? not
• Which lessons is this?

Also, Turkish often keeps nouns singular after numbers, so singular forms are very common in these kinds of structures.

A rough English-friendly approximation is:

kach-un-juh

A few important sounds:

• ç = ch as in chair
• ı = a vowel that does not exist exactly in English; it is a dull, central sound, not ee
• c in Turkish is pronounced like j in jam

So kaçıncı sounds approximately like: kach-uhn-juh

And the full sentence is roughly: boo kach-uhn-juh ders

You answer with an ordinal number:

• Bu birinci ders. = This is the first lesson.
• Bu ikinci ders. = This is the second lesson.
• Bu üçüncü ders. = This is the third lesson.

You can also make it more specific:

• Bu üçüncü dersimiz. = This is our third lesson.

So the basic pattern is:

Bu + ordinal number + ders.

Yes, if the context is already clear.

You might hear:

• Kaçıncı ders?

This is shorter and can work in conversation when everyone already knows what is being discussed.

But if you want the full, clear sentence, use:

• Bu kaçıncı ders?

Yes, depending on what you want to point to:

• Bu kaçıncı ders? = Which lesson is this?
• Şu kaçıncı ders? = Which lesson is that? (often something near the listener or being pointed out)
• O kaçıncı ders? = Which lesson is that over there / that one?

So the structure stays the same; only the demonstrative changes.

They ask different things.

• Bu kaçıncı ders? = asks about order/number in a sequence

  • first, second, third, etc.

• Bu hangi ders? = asks which class/subject/course

  • math?
  • history?
  • Turkish?

So:

• Bu kaçıncı ders? = What number lesson is this?
• Bu hangi ders? = What lesson/subject is this?

That is one of the most useful distinctions to learn.