That's right. If a noun is plural, also the corresponding adjectives (including possessive pronouns, which are also adjectives) have to be. Same if the noun is in the accusative. The adjectives mirror them.
Verbing adjectives will give you "to be [adjective]" but verbing nouns will give you "to use [noun]". It's like in English when you hammer something. You're saying that you're using a hammer, not that something is a hammer.