Какие времена надо поставить в предложениях: the teaching assistent ( explains/explaining)to students how to use the new microscope. what (you/look) at? -the sky.it (look) as if its going to rain.the famous opera singer(apptar) ft opera house tonight.

1. The teaching assistent is explaining to students how to use the new microscope. (Pr. Contin.)
2. What are you looking at? (Pr. Continuous)-The sky.It looks (Pr. Simple) as if its going to rain.
3. The famous opera singer (apptar) ft opera house tonight - Здесь у вас ошибка в написании глагола. Скорее всего, здесь будет is appearing (Pr. Cont)

Is explaining- what are you looking at - looks - is + глагол с окончанием ing (вы неправильно вбили написание, поэтому трудно понять что это за глагол)