When my husband and I were living in the Netherlands, random strangers on the street would ask us why we spoke English with each other – and then told us that we should stop doing it, because it would prevent us from learning Dutch. The same advice is, of course, often given to migrants by well-meaning politicians, schoolteachers, shop assistants, strangers on the train and other language experts.
Why does it not work like that?
Imagine that you wanted to become a long-distance runner. If you were sensible, you would pick your training moments, and also choose your off-times. What you would not do is try to run during every waking moment – this would not make you a better runner, it would make you a very bad runner who had developed all kinds of bad habits.
If my husband and I had spoken Dutch with each other, we would inevitably often have made mistakes, and the mistakes would then eventually have become part of our grammar. Remember the lovely German couple in the movie Casablanca, who were speaking only English to each other to prepare for their emigration to America? Their conversation runs “What watch?” – “Ten watch.” – “Such much?” (translation: what time is it – ten o’clock – so late?) One can imagine the kind of English they would be speaking by the time they got to the States.
Learning a second language is about the quality of the input you get, not about the quantity.