The goal of every language teacher should be to create optimal conditions for learners to acquire the language. However, opinions on language teaching and also on teaching grammar in particular are very varied and often contradictory. In order to create optimal learning conditions, it is necessary to understand how language teaching influences language acquisition. The specific question which underlies this paper is if and how much explicit grammar teaching should take place in the second language classroom. This very broad question entails several other questions. One of these is if second language learners can acquire a structure by learning explicit rules, but also if they can acquire a new structure without being explicitly made aware of it. This paper tries to find out about the interrelations of implicit and explicit knowledge and about the development of the two kinds of knowledge depending on the time of instruction.