Выделите сказуемое, определите его видо-временную форму и укажите инфинитив : what are they eating now? they usually go to the river for a swim. he spent last summer in the country. will you go to the south next summer? i was going to the cinema at four o'clock yesterday. my mother likes to read books. have you ever spent your holidays in the crimea? he will be writing a letter at 5 o'clock tomorrow. i shall have done my work by seven o'clock tomorrow. she had drawn this picture by 7 o'clock yesterday.

What are they eating now? are eating - present continuous - to eat
They usually go to the river for a swim.  - go - present simple - to go
He spent last summer in the country. - spent - past simple - to spend
Will you go to the south next summer? - will go - future simple - to go
I was going to the cinema at four o'clock yesterday. - was going - past continuous - to go
My mother likes to read books. - likes - present simple - to like
Have you ever spent your holidays in the Crimea? - have spent - present perfect - to spend
He will be writing a letter at 5 o'clock tomorrow. - will be writing - future continuous - to write
I shall have done my work by seven o'clock tomorrow. - shall have done - future perfect - to do 
She had drawn this picture by 7 o'clock yesterday. - had drawn - past perfect - to draw