所以我下定决心要学好英语用英文怎么说

"I have made a firm resolution to learn Engpsh."

来源：下定决心 (to make a firm resolution) 是一个固定短语，表示决定做某事的意思。

英文例句：

I have made a firm resolution to exercise every day.
She made a firm resolution to quit smoking.

中文翻译：

我下定决心每天锻炼。
她下定决心戒烟。