Other Germanic roots in English and other Western Germanic tongues

Being German, I always look for alikeness in English and German words when I think about Anglish before the Normanns. But I stumble over some word groups where this knowledge is not enough to explain why we say the things we say. For example: Purchase (latinised English) - buy (English) - kaufen (German ) or kopen (Dutch) Inquire- ask - fragen or vragen