Questions & Answers about Latem najczęściej biegam rano w parku.
Latem is the instrumental case of lato used adverbially to mean “in summer / during the summer”.
Polish often uses the instrumental case without a preposition for general time expressions, especially seasons:
- latem – in (the) summer
- zimą – in (the) winter
- wiosną – in (the) spring
- jesienią – in (the) autumn / fall
You can also say:
- W lecie najczęściej biegam rano w parku.
That is also correct and means the same, but latem is shorter and more idiomatic in this kind of sentence.
Plain lato at the beginning (without case ending, like Lato najczęściej biegam…) would be wrong here, because lato as a bare noun is not in the right case for this time expression.