What's the difference between can't and mustn't? Are both ok in the following sentence? Thank you!
You mustn’t/ can't take heavy clothes when you go to Bangkok where temperatures are very hot all the year round.
"Mustn't" means that something is forbidden, against the law, against your religion, or something like that. You may not need heavy clothes in Bangkok, but if you have them with you for a different part of your trip, there is no reason why you must leave them outside the city.
"Can't" means you are not able to do something. People are able to take heavy clothes to Bangkok; they just won't have any use for them there.
I would use "shouldn't," or perhaps "don't need to." Both of these are milder.