Verbs need subjects, which can be nouns or pronouns.
Verbs have their own part of speech.
Verbs
Can and relax are verbs.
Did and attend are verbs, and not is an adverb.
Did and know are verbs. Not is an adverb.
They are verbs.
Action words are verbs. Verbs don't describe another part of speech. The show the action, state of being, or occurrence in a sentence. Adverbs are words that describe (modify) verbs.
Some verbs indicate existence.
Adjectives.
The word adverb is a noun. However, adverbs (the words that modify verbs, adjectives, and other adverbs) are their own part of speech.
Parts of speech are like nouns, verbs, adjectives, article, pronoun etc.
"Could" is part of a special group of verbs known as modal auxiliary verbs. Some other modal verbs are may/might, will/would, shall/should.