1. Larry was playing computer games while his brother chat with a friend yesterday evening.

1. Larry was playing computer games while his brother
Разберем предложение. Необходимо вставить пропущенное слово в правильной форме. Глагол chat нужно поставить в форму Past Continuous. Так как брат один, то необходимо использовать форму was chatting.

Larry was playing computer games while his brother was chatting with a friend yesterday evening.

Ответ: was chatting