Questions & Answers about W tygodniu zwykle jem obiad w pracy.
W tygodniu literally is in the week, but idiomatically it means on weekdays / during the week (as opposed to the weekend).
- w = in
- tydzień = week (dictionary form, nominative)
- tygodniu = locative case of tydzień
After the preposition w meaning location or time (in, on, during), Polish usually uses the locative case, so tydzień → tygodniu.
Polish has no articles (no a, an, the at all).
Whether English would use a or the is understood from context, not from a separate word:
- W tygodniu → during the week / on weekdays
- jem obiad → I eat (my) lunch / I have lunch
There is no way to mark definiteness directly; you just rely on context.