.I am having a bit of trouble finding any rules about the past and future tesne of must. Using the word must in a future tense and past tense sentence is easy enough..
He must go to the doctor. (f) This is not a future tense sentence.
He must have gone to the doctor. (p) This is not a past tense sentence.
The word must changes it's tense with the use of auxiliary verbs.
Is it as simple and as straightforward ("straightforward" is one word) as that? No. Do I need to explain that the negative of must is 'mustn't'?
Any advice on how to explain this to EFL students would be great. Thanks.