Function of adverb clause

[Jupiter]

Hi teachers,

Please look at this sentence.

If you want to write well, you must pratise.

What does the adverb clause modifiy?
Does it modify must pratise or pratise? Or it modifies the whole sentence?

Thanks in advance

Jupiter

[Mister Micawber]

.
It modifies the whole main clause.
.
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Native English teacher at Mister Micawber's