I know that the two mentioned abilities are different. I always find it quite straightforward to understand a foreign language when reading. provided I have a good knowledge of its grammar and a large vocabulary. But when it comes to speak it, so, to create a new, personal and creative content, I may experience an added difficulty. The words I look for in order to use them may not immediately come to the mind, even though when I read them I don't have to think about their meaning.

So, is there any psychological reason why is it less difficult to understand then to speak, even when the same vocabulary and rules are involved?

Of course, I may be wrong in this claim.