Possible answers:

1. one (cardinal number), first (ordinal number)
2. a solution (noun), to solve (verb)
3. to complain (verb), a complaint (noun), complaining (adjective)
4. constant (adjective), constantly (adverb), constancy (noun)
5. they (personal pronoun), their (possessive pronoun), them (objective pronoun)
6. to commute (verb), a commuter (noun)