You had a little typo. All the words contain "fordern"... fordern: To ask somebody for something but in a very insisting way (actually more claiming than asking) Er forderte eine bessere Bezahlung. He insisted on a better payment. anfordern: Very similar to "fordern" but more concerned with asking a good or help to be delivered. Der Polizist forderte Unterstützung an. The policeman asked for support (calling for other policemen). auffordern: To request somebody to do something. Der Polizist forderte den Fahrer auf, aus dem Auto zu steigen. The policeman asked the driver to leave his car. erfordern: A erfordert B = A requires B Ein erfolgreiches Studium erfordert viel Energie. It requires a lot of energy to study successfully.