Я забыл, какой это предлог.

Because какой это предлог is a subordinate clause: an indirect question embedded inside the main sentence.

• Main clause: Я забыл = I forgot / I’ve forgotten
• Subordinate clause: какой это предлог = what preposition this is

Russian normally puts a comma before this kind of subordinate clause, just as English often separates similar ideas with that / what / which structures.

Забыл is the masculine singular past form of забыть.

A few useful points:

• забыть is perfective, so забыл often means forgot or have forgotten
• In this sentence, it means the speaker does not remember at the moment

So Я забыл can sound like:

• I forgot
• I’ve forgotten
• I can’t remember

The exact English translation depends on context.

Then it would be:

Я забыла, какой это предлог.

Russian past tense changes for gender in the singular:

• забыл = masculine
• забыла = feminine
• забыло = neuter
• забыли = plural

So a woman would normally say забыла, not забыл.

Какой means what kind of / which and is used when asking about the identity or type of something.

Here, the speaker is asking: What preposition is it?

So:

• какой это предлог? = What preposition is this?

Using что would not work naturally here, because что means what, but not in the sense of choosing or identifying a type in this structure.

This is a very common question.

In Russian:

• какой = what kind / which one (in terms of identity or type)
• который = which one from a specific known set, or sometimes what number / which in order

Here the speaker is trying to identify the preposition, so какой is the normal choice:

• Я забыл, какой это предлог.

Using который here would sound wrong or unnatural in standard Russian.

Here это is part of the very common identifying pattern:

• Это X = This is X / It is X

So:

• какой это предлог? literally = what preposition is this?

In the full sentence:

• Я забыл, какой это предлог = I forgot what preposition this is

The word это helps point to the item being identified. It often appears in sentences where you name or classify something.

Because inside the subordinate clause, предлог is not the object of забыл.

The structure is:

• Я забыл = main clause
• какой это предлог = whole subordinate clause acting as the thing forgotten

Inside that subordinate clause:

• это = this / it
• предлог = predicate noun, naming what this is

That is why предлог stays in the nominative:

• Это предлог. = This is a preposition.

So the speaker did not literally forget a preposition as a direct object in the grammar of the sentence; rather, they forgot which preposition it is.

Russian word order is somewhat flexible, but this sentence in its neutral form is:

Я забыл, какой это предлог.

That is the most standard and natural order.

You can sometimes shift things for emphasis, but not every rearrangement sounds equally good. For a learner, it is best to keep this standard pattern.

Related direct question:

• Какой это предлог? = What preposition is this?

That direct-question order is also standard.

Yes. That is a very natural alternative.

• Я забыл, какой это предлог.
• Я забыл, что это за предлог.

Both mean roughly the same thing: I forgot what preposition this is.

A small nuance:

• какой это предлог sounds a bit more straightforward and grammatical-school style
• что это за предлог is very common in everyday speech and can sound a little more conversational

Both are correct.

No. Предлог can also mean pretext or excuse in other contexts.

For example:

• под предлогом = under the pretext of

But in this sentence, because of the learning/grammar context, предлог clearly means preposition.

No. It contains an indirect question.

Compare:

• Direct question: Какой это предлог? = What preposition is this?
• Indirect question: Я забыл, какой это предлог. = I forgot what preposition this is.

In Russian, indirect questions are usually written like normal statements, not with a question mark.

Not in this sentence.

Russian забыть can sometimes mean to forget in the sense of failing to take or bring something, but here the rest of the sentence makes it clear that it means to forget mentally / not remember.

So here:

• Я забыл, какой это предлог = I don’t remember what preposition this is

not

• I left the preposition somewhere

This sentence is already natural, but in conversation Russian speakers might also say things like:

• Я забыл, какой тут предлог. = I forgot which preposition goes here.
• Я не помню, какой здесь предлог. = I don’t remember which preposition is used here.
• Какой тут предлог? = What preposition goes here?

These versions are especially common when talking about which preposition fits in a phrase or sentence.