1. Imagine it is lunch time now.

2. Imagine it is a lunch time now.
(which one is right? do we have to use 'a' or not?)

3. Suppose he is a stranger. And answer the question.

4. Imagine he is a stranger. And answer the question.

5. Pretend he is a starnger. And answer the question.

6. Think he is a stranger. And answer the question.

(Are they all the same and grammatical?)

1 is correct; 2 is incorrect. The word "a" is not used here.

3, 4, and 5 are fine; 6 is not.