Learn Russian: Дети могут заиграться во дворе и забыть о времени. - Children can get carried away playing in the yard and forget about the time.

The prefix за- combined with the postfix -ся forms a pattern meaning to do something to excess or become completely absorbed in an action. Added to играть (to play), it creates заиграться, meaning to play for too long or get carried away playing.

Since playing is a continuous action, why is заиграться in the perfective aspect here?

While playing itself is an ongoing process, this specific verb focuses on the result of crossing a boundary into excess. The sentence describes a potential completed outcome (they might cross that line and forget the time), so the perfective infinitives заиграться and забыть are used after могут.

Why is the preposition во used before дворе instead of the usual в?

The preposition в changes to во before words starting with certain tricky consonant clusters, especially those beginning with в, ф, or combinations like the дв- in дворе. This is simply to make the phrase easier to pronounce.

Why does the word время (time) look like времени at the end of the sentence?