I enrolled in italki I hope to study writing English. Before that, I have to study an English grammer book.

fractaluju

교정 · 2

I ~~enrolled~~ in Signed Up/Created an Account on Italki [1]

I hope to study writing English. Before that, I ~~have to study~~ had studied / had been studying with [2] an English grammar book.

Enrollment is used for classes where there is a set curriculum, like university. In this case, italki is a platform for learning languages, so "signed up" or "created an account" is appropriate.
"Have to study" implies that you were forced to study the grammar book.

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