How do we use the modal verbs must and can?

[Ricardos]

English Grammar Error, Intermediate level

ESL/EFL Test #4 "When you go to France...", question 3

In that case you must can to speak the language as well as a native French person.

(a) can
(b) speak
(c) as well as
(d) French

English Grammar Error, Intermediate level

ESL/EFL Test #4 "When you go to France...", answer 3

In that case you must be able to speak the language as well as a native French person.

Correct entry: be able
The error was: (a) can
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why can?

[Mister Micawber]

.
'Can' is wrong in the original sentence, and must be replaced by 'be able'. 'Must can to' is not an allowable English structure.
.
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Native English teacher at Mister Micawber's