There are two steps to conveying a thought in a language:
1. recall the grammar rules and vocabulary 2. form the sentence
The steps are fundamentally different in that the first one is an exercise in recollection whereas the second one is a "creative" process of some sort.
To prove a mathematical fact you go through the same two steps:
1. you recall all the past mathematical knowledge 2. you deduce the fact in a creative process of meshing together the facts you just recalled
So conveying a thought in a language is sort of analogous to proving a mathematical fact.
But clearly when doing math step 2 is given the spotlight whereas when you are talking in a foreign language step 1 is by far the hardest.