Student or Learner
A: I've got a bit of a headache.
B: I think you must / should take an aspirin.
I think we can use either one of the two. What do you think? I feel a little bit confused.
Thanks a lot!
"Must" is a very definite statement so I wouldn't use the uncertain "I think" with it.
I think you should take an aspirin. (Giving your opinion)
You must take an aspirin. (Giving an order)
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.