Questions & Answers about Летом мы поехали в маленький порт на море.
Летом comes from the noun лето (summer) and is in the instrumental case. In Russian, the instrumental is often used without a preposition to express time:
- летом – in (the) summer
- зимой – in (the) winter
- весной – in (the) spring
- осенью – in (the) autumn/fall
So Летом мы поехали… literally is like “By summer / in summer we went…”.
You cannot say ✘ в летом. To say “this summer”, you use an instrumental phrase like:
- этим летом – this summer
- прошлым летом – last summer
So the pattern is: [instrumental form of the time word] without a preposition.