the difference between 'must' and 'should'?

[Diemthuydiem]

hi every body! :P
In a letter of credit, if it says: 'commercial invoice is made in 3 fold(s) and should show letter of credit number'
I don't understand if it forces to show letter of credit number on commercial invoice or not? or it depends that i like or don't like to show it.
and i don't understand why my manager said that the sentence 'commercial invoice is made in 3 folds' is wrong. it must be said 'commercial invoice is made in 3 fold', we can not add 's' behind the word 'fold' in this case.
please help me to explain these problems.
thanks,
DIEM

[Mister Micawber]

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In this case, 'should' = 'must' (that is, both tell us that the L/C number is required). 'Should' is more polite, more diplomatic.

In my English '-fold' is no longer a common suffix. I would say 'the commercial invoice in made in triplicate'.
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Native English teacher at Mister Micawber's

[Diemthuydiem]

thank you for your explaination. But when i issue a letter of credit for my customer, sometimes they want me to use the phrase 'in 3 fold' instead of ' in triplicate'. so please tell me the right way to use it. i'll write 'in 3 folds' or 'in 3 fold'. In this case, 'fold' is a non-count noun or count noun, and if it's right that plural form of 'fold' is 'folds' or wrong?
thanks,

[Mister Micawber]

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No -s.
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Native English teacher at Mister Micawber's