What part of speech is “could”

'could' is a past form of the verb 'can' and is mainly used as a verb to refer to a potential action that may take place. When used as a verb it is a modal verb that is used to express potential or ability in either the present or past. It is often used to express a polite request or query to another.

1. 'She could fly across the country overnight.'

2. 'Could you help me out with this task?'

3. 'He could win the championship if he tried his hardest.'

'could' is a gentle and polite alternative for 'can.' It is often used to make a request more gentle and to show politeness and respect. Additionally, the verb form of 'could' is often used when expressing an opinion that something might be possible.