Modal verb: must

[Adolf]

Common Errors in English, Intermediate level

ESL/EFL Test #2 "Between the lines", question 2

I honestly can't remember how long I've been waiting but it must have been since at least 45 minutes.

(a) honestly
(b) I've been waiting
(c) must
(d) since

Common Errors in English, Intermediate level

ESL/EFL Test #2 "Between the lines", answer 2

I honestly can't remember how long I've been waiting but it must have been for at least 45 minutes.

Correct entry: for
The error was: (d) since
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why you used it +must have

[Alan]

Hi,

The use of must expresses certainty. She must have missed the bus = it is clear that she has missed the bus.

Alan
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English as a Second Language
You can read my ESL story Phrasal Verbs/take