Is "any" singular or plural? eg: I don't have any book. or I don't have any books. Which is correct?

There is a difference: The first one is present progressive - the action is not finished. The second one is present perfect - The action is just over.

There may be a difference. Thought is intentional. But occurence is not intentional.

Usually both to & for are used with essential , like Food is essential to all animals. Food is essential for all animals. But in question 23, for looks more suitable because of the following part to function competently .

As suggested by some of us, according to the rules, it should be: She said that she was risking everything she had for me.

If you want, if you feel like, etc..

Agree with Clive

It can be omitted if 1) it is clear in the context - no need 2) it is of no significance - no use

The meaning is not clear. 50 mm can be the length of the pipe. What is10% pipe? What does 'depth' refer to? Is the 'wall thickness' that of the pipe?

There are two methods. Q: Who doesn't live in Spain? A: I = I don't live in Spain Q: Who doesn't live in Spain? A: I don't = I don't live in Spain