Questions & Answers about Я недолго жду друга у входа.
Друга is in the genitive singular case.
The verb ждать (to wait) normally governs the genitive in standard Russian, so:
- ждать друга – to wait for (one’s) friend
- ждать автобуса – to wait for the bus
- ждать поезда – to wait for the train
With masculine animate nouns, the accusative and genitive forms look the same in the singular (друга), so you can’t see the difference just from the ending. But grammatically, here it is genitive because of ждать.
With inanimate masculine nouns, you can see the genitive more clearly:
- Nom.: поезд
- Acc.: поезд
- Gen.: поезда
ждать поезда – to wait for the train (here you clearly see the genitive ending).