The main verb is always the bare infinitive
COULD and COULDN'T are the past tense forms of CAN and CAN'T.
COULD and COULDN'T refers to ability or inability in the past.
We use COULD to mean that we were able to do something in the past.
Ex: When she was 1, she could walk.
We use COULDN'T to mean that we weren't able to do something in the past.
Ex: When she was 1, she couldn't speak.
For each sentence, choose between COULD and COULDN'T.