Once you__smoking,it is hard to __.
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Once you__smoking, it is hard to __.

这里的意思是说

一般你养成吸烟的习惯,再想戒掉就相当难了.

get into the habit of doing something 是固定用法,形成...习惯

而used to为过去常常做某事

have become used to是已经习惯了去做什么事情,并不等于养成习惯

这里主要强调有一个过程

are used to显然不合适