intransitive/transitive to have learned or found out about something‘Have they arrived yet?’ ‘I don’t know.’
If you don’t know the answer, just guess.
know (that): I knew she wasn’t really happy.
know what/where/how etc: I don’t know where the money’s coming from.
know if: I need to know if she’s made a decision yet.
know something about someone/something: I don’t know anything about it.
know something for sure/for certain/for a fact: We know for a fact that the fire was started deliberately.
know something from experience: We know from experience that unemployment makes the problem worse.