SHOULD and MUST
We use SHOULD to point out a good choice or option:
You should get your wife some flowers for Valentine’s Day.
We use MUST for a situation where you have no choice:
All visitors must sign in.
Although the two words seem similar, there are many situations where only one of them can be used:
Wrong: You should get a visa before you visit China.
Right: You must get a visa before you visit China.
Wrong: You must recycle that plastic.
Right: You should recycle that plastic.