Is grammatically correct?

Question

Why it's hard to understand in practice? 

Answer 1

Favourite answer

Is THIS grammatically correct?

No.  This is:   Why IS IT hard to understand in practice?

Answer 2

More a declaration than a question, although you might be able to say it in a question tone and the listener would get the idea.  you haven't reversed the subject-verb order to make it into the form of a question, so it isn't actually a question, unless you can use tone or other means to indicate that you mean it as a question.

Its form is that of a declaration, and an incomplete one at that.  no antecedent for "why".

Answer 3

No, sorry, it is not correct, and I am not sure what the context is, but I will try to help.

You could say:

The theory of chemistry is easy to understand, but it is the practice of chemistry that is hard. 

The theory of chemistry is easy to understand, but in practice, chemistry is hard. 

**Used chemistry just as an example. 

Answer 4

Yes. It is fine as written.

Answer 5

No. There is no subject.