I was telling a student something i ve put into words just recently.
The sense of achievement students get when they build a sentence in English in a normal communication situation, just chatting with me for example. Word by word, little by little, slow, conscious production.
That's what we don't really want in a way, we want fluency over accuracy, we want them to connect things but i find it hard for them to learn in chunks.
Also the fact that they sometimes stop and seek reassurance or feedback after saying two words, as if waiting for you to approve this before they continue. What they say may be correct or not, depending how they continue the sentence... they don't get that
third is the fact that they sometimes think that we need xyz depending on the tense we use.
that applies to time references/adverbs/time adverb phrases but not to prepositions, ...