1. He brushes teeth in the evening.
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?)
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.
The difference between sentences #1 and #2 lies in the use of the indefinite article "the" before the word "evening." Sentence #1, "He brushes teeth in the evening," suggests that he brushes his teeth at a specific time in the evening, perhaps as part of his daily routine. On the other hand, sentence #2, "He brushes teeth in the evenings," implies that he brushes his teeth during multiple evenings, but the specific time or frequency is not explicitly mentioned.
Similarly, the difference between sentences #3 and #4 is the use of the modal verb "will" before the verb phrase "brush teeth." Sentence #3, "He will brush teeth in the evening," indicates a future action where he is going to brush his teeth at a specific time in the evening. In contrast, sentence #4, "He will brush teeth in the evenings," suggests a future action where he will be brushing his teeth on multiple evenings, without specifying the exact time or frequency.