Must and must be

Question

Are they interchangeable?1. Students in my class study very hard. So they must be smart.2. Students in my class study very hard. So they must get or be getting high grades.Are they ok? thank you

Avangi · Accepted Answer

mr1binAre they interchangeable?
1. Students in my class study very hard. So they must be smart.
2. Students in my class study very hard. So they must get or be getting high grades.

"Must" and "must be" are not generally interchangeable.

The two options in your second example are two different tenses.

"Be" functions as an auxilliary in the continuous tenses: They must be getting high grades.

Aside from the modal "must,"

"He gets high grades," is simple present. (habitual behavior)

"He is getting high grades" is present continuous. ("Is" is the auxillary "to be.")

When you switch to the modal version, "is" becomes "be."

"He must get high grades."

"He must be getting high grades."

The main finite verb in your second example is "to get." The verb "to be" functions as an auxilliary.

The main finite verb in your first example is "to be."

"He is smart" when switched to the modal becomes "He must be smart."

In these examples, the collocation "must be" is just coincidental.

In other cases, it can stand alone as a casual reply:

He's driving a new car. Maybe he got a raise. (reply) Must be!

Must and must be

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