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

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fractaluju

التصحيحات · 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.

انضم لمجتمع التعلّم هذا وجرّب التمرينات المجانية!