1. He brushes teeth in the evening.

Question

2. He brushes teeth in the evenings.
3. He will brush teeth in the evening.
4. He will brush teeth in the evenings.

(What is the difference between #1 and #2, and #3 and #4?)

Answer 1

All are OK, but they'd all be better if you wrote "brushes his teeth" in all four.

There are no differences in meaning in the two pairs of sentences.