私は、８時になったら、英語の勉強をするつもりです。

I will start/ begin to study English at 8am. 用 英語 (美國) 要怎麼說？

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You can use "begin to" instead of "start."

You could also say it like this:
"I'll start studying English at 8." In this case, you could also say "I'll begin studying," but that sounds a little less natural.