Learn Polish: Kiedy grają w berka, jeden chłopiec goni, a dziewczynka ucieka i krzyczy. - When they play tag, one boy chases, and the girl runs away and screams.

When talking about playing a game or a sport, Polish uses grać w plus the Accusative case. Berek (tag) is a masculine noun. Even though it is not a living person, names of games and sports often take the masculine animate ending -a in the Accusative, making it w berka.

Why use a instead of i before dziewczynka ucieka?

In Polish, a is a contrastive conjunction. While i simply adds things together, a emphasizes a difference or shift in focus between two subjects. Here, the boy's action (chasing) is being contrasted with the girl's action (running away).

Are goni and ucieka imperfective or perfective verbs?

They are both imperfective verbs. They describe the ongoing, continuous process of chasing and running away, which makes sense here because they are in the middle of playing the game.

Could I use gdy instead of kiedy at the beginning of the sentence?

Yes, kiedy and gdy both mean when and are completely interchangeable in this context for introducing a time clause.