An example of useful used as an adjective is the phrase "useful information" which means information that is helpful.
- Having a beneficial use; serviceable: a useful kitchen gadget.
- Being of practical use: a useful job; useful members of society.
- Prepositions: useful is used in useful for <purpose>, useful for <person> and useful to <person>. The words useful to are also found in construction such as It is useful to do, in which to marks an infinitive rather than being a preposition.
From use +"Ž -ful.