Hi!
When do you say at night and when to say in the night?
also When do I say in summer winter  and when do I say in the summer/winter?
And why do some people say fish when there are many and others say fishes which one is correct?
thanks

1. I always do exercise at night (when night time comes).

2. I always do exercise in the night (usually in the dark, with little light).

3. I always do exercise in the summer. Some people leave out "the" for conversational purposes.

4. "Fish" is used as a whole, not matter how many species are there; whereas

    "Fishes is used when one wants to name the species.