When I'm teaching French, sometimes a child will ask me for a word or a phrase that they haven't learned yet and my automatic response is 'just use the words you know'. On occasion, this is because I can't think of the word, or have never learned it (my daughter had to teach me the French for mobile phone!) but often I do know and yet for some reason my automatic reaction is to be unwilling to tell them. Why? I tell them new words all the time, why should I be stopping myself from telling them one more. I did wonder if it was pure laziness on my part: if I start by answering this question, will I then be bombarded with vocabulary requests that I can't answer?
I don't think it is that. I think it has to do with learning how to use language, something that becomes evident when you try to speak a foreign language (and perhaps in those brain conditions where one begins to lose words). In our native language, we don't use words we don't know. We pick up new words by reading them or hearing them, not by going up to someone and saying 'what's the word for an orangey brown animal with a long neck that lives on the plains of Africa?' The word and the concept that goes with it arrive together. Even using a thesaurus, the word you choose ought to be a word you already know, or how can you be sure the context is right?
So the difficulty with a foreign language is that there are concepts in your brain for which you have no words. The children's answer to this is 'provide me with the words', but in the real world, part of speaking a foreign language is gaining the ability to work around lack of vocabulary, learning how to make yourself understood using those words you do have. I remember in France once I took a phone call from a boiler supplier. He really needed to be talking to the builder working next door and as I attempted to rustle up the words to tell him so, I could hear the little-used synapses in my brain firing up. I managed to produce 'chef de construction' (building boss?) which did the trick, but certainly wasn't what a French person would have said!
Then there's the question of words and phrases that exist in one language but not in another. Take the word 'excited'. Children in my class are always wanting to tell me they are excited about things. French children too must get excited, but perhaps they just don't talk about it, because there is no obvious word or phrase that captures the essence of anticipation and enthusiasm. It's hard enough to explain to the concept of gender to British children, this idea of words not necessarily being translateable is an even bigger one to swallow.
I suppose what I'm getting at is that part of what they need to learn is to switch off their English-speaking brain and start using their French. I know this is very hard when you only know a very little, but I suspect that it's the best way to turn a bunch of random words and phrases into a vocabulary.