Проверьте пожалуйста:
Закончите следующие предложения. Используйте can,t или couldn,t и один из глаголов. Eat decide find gо sleep
a) I was tired but I couldn,t sleep.
b) I wasn,t hungry yesterday. I couldn,t ate my dinner.
c) Ann doesn,t know what to do. She can't decide.
d) I wanted to speak to Martin yesterday but I couldn,t found him.
e) Jim can't go to the concert next Saturday. He has to work.
f) Paula couldn,t went to the meeting last week. She was ill.

Почему глагол не меняется?

Ведь время прошедшее ( я не съел свой ужин).

could - прошедшая форма can, can - модальный глагол, после модальных глаголов идет инфинитив без 'to'

Спасибо большое, теперь понятно стало.

couldn't eat my dinnercouldn't find himcouldn't go

can't sleep can't go couldn't ate my dinner