答案：I forgot【to lock 】the door,i'll go back for it.
翻译；我忘记去锁门了,我将为此返回来.
语法；forget to do sth .翻译为忘记去做某事.
forget doing sth .翻译为忘记做过某事.
解析；此题是忘记去锁门,所以要要填to lock.
不明白及时追问,
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