Question

She ..........Work hard if she wants to top the class.

Answer 1

She MUST work hard if she wants to top the class

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Answer 2

She must Work hard if she wants to top the class.

In general, the words “should” and “must” are  considered as the modal auxiliary verbs or  these are also known as simply modals.  

These kinds of verbs provide information about the function of the main verb following it.

Of course, both must and should are similar.