,,leugnen'' ,,abstreiten'' and ,,bestreiten'' is nearly the same. You can use all these three words for example if you get accused for something, maybe a crime, then you can leugnen/abstreiten/bestreiten that you have been the one who committed the crime. So I'd say you can use it, if someone gives a statement (especially based on you) and you don't agree with it. ,,Ablehnen'' is normally used if someone offers you something (for example a present) and you reject it. ,,Verneinen'' can mean, that someone answered a question with ,,no'' (por ejemplo: ,,Er hat die Frage verneint'' is the same as ,,Er hat die Frage mit ,Nein' beantwortet'') Hope I could help! :)