Something really interesting happened during a lesson last week. I played a cassette to a student to test his understanding of spoken English. On the cassette, a lady said, "I won't be back until eight O'clock". I stopped the cassette, and asked the student what she said. Quite confidently, he replied, "she said she will return home at eight O'clock."
He had by and large understood the message. But he couldn't recall the words used. This is because an expression like "I won't be back until eight" doesn't figure in his active vocabulary, and can't be translated literally into French. He nevertheless got the gist.
This shows the gap between what we understand and what we are able to produce. I don't know how teachers could accelerate the process of transforming passive understanding into active usage. After all, that should be our job. It seems to me that massive exposure to language along with lots of practice is the only solution, but someone studying two hours a week with little time for homework is surely not going to make rapid progress.