You'd be surprised. Grammar might not work that way, and it doesn't, but vocab and grammar are learnt differently. A lot of people (not all, because there are as many ways of learning as there people) learn vocab first and then learn grammar constructs separate. In the stage where you are internalising vocab it is very easy to do the direct word substitution. Thankfully, oh god thankfully, that tends to vanish once the grammar constructs kick in.