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.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber