"下决心戒烟"用英语可以表达为"make a resolution to quit smoking"。

这个表达方式的来源是由两个单词组成的，分别是"make a resolution"和"quit smoking"。

"make a resolution"是指"下决心，做出决定"的意思。

"quit smoking"是指"戒烟"的意思。

下面是一些英文例句及其中文翻译：

I made a resolution to quit smoking last year. (我去年下决心戒烟。)
She made a resolution to quit smoking and has been smoke-free for six months. (她下决心戒烟，已经戒烟六个月了。)
He made a resolution to quit smoking and it was the best decision he ever made. (他下决心戒烟，这是他所做过的最好的决定。)