1.The mouse ate a bit of the cheese.

2.The mouse ate a bit of cheese.
(Which one is correct? Are both OK?)

3. Man lived in caves long,long ago.
4. Men lived in caves long,long ago.
(Which one is correct? Are both OK? What is the difference between 'man' and 'men' in the sentence?)

2 is better than 1.

In 3 and 4, 'man' really means 'mankind' or 'humans.' In reality, both are correct, but it would be more accurate to write 'humans.'