Questions & Answers about Летом я отдыхаю на даче.
Летом is a very common Russian way to express time, and here it functions like an adverb meaning in summer / during the summer.
Russian often uses forms like this with seasons and parts of the day:
- зимой = in winter
- весной = in spring
- летом = in summer
- осенью = in autumn
- утром = in the morning
- вечером = in the evening
Historically, this is connected to the instrumental case, but for a learner it is often easiest to remember летом as a fixed time expression meaning in summer.
В лето is not the normal way to say in the summer in modern standard Russian.
It is the instrumental singular form of лето.
The noun is:
- nominative: лето
- instrumental: летом
In this sentence, though, it is not being used as a normal object. It is being used adverbially to express time when something happens.
So:
- Летом я отдыхаю... = In summer, I relax / spend time off...