By what?

[HSH1985]

Can we say "by what"? like "by which".
if not,what's the preposition of what?

I've a lot of problems with the prepositions,what should i do?

[Raymott]

Can we say "by what"? like "by which".
if not,what's the preposition of what?

I've a lot of problems with the prepositions,what should i do?

You need to put those phrases in a sentence so we know what you mean.
We don't say "By which" by itself, and nor do we say, "By what".
Some context is needed.

[HSH1985]

thanks a lot.
and can we use "by what" in the sentence?

[5jj]

thanks a lot.
and can we use "by what" in the sentence?

Yes.

That is not exactly a helpful answer, because one can create situations in which almost any two words in English can appear next to each other.

As Raymott suggested, try putting the words in a sentence in which you think they might be possible, and we'll tell you if you are right.