What is the difference between "must" and "should"?

[Jorien]

Test No. incompl/elem-16 "Learn it by heart", question 3

I think learning ......... be interesting because otherwise you wouldn't ever be able to succeed.

(a) can
(b) should
(c) must to

Test No. incompl/elem-16 "Learn it by heart", answer 3

I think learning should be interesting because otherwise you wouldn't ever be able to succeed.

Correct answer: (b) should

Your answer was: incorrect
I think learning must to be interesting because otherwise you wouldn't ever be able to succeed.
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I still do not understand why 'can' and 'must to' are wrong in these sentence. Are they only wrong because of the meaning, or is it really strange to say such a sentence?

[Torsten]

After must we don't use to. Should is the correct choice in this sentence because the speaker is making a suggestion.

Learning can be interesting. (it is possible that learning is interesting)

Learning should be interesting. (I want learning to be interesting)
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary

[Simon]

[Torsten]

It simply means that if learning is interesting than you will succeed.
Let me know if this makes sense to you or if you wanted to know something else.
As for the word otherwise - it is used to describe two different situations.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary