同学你好，该知识点来自沪江网校《英语零基础直达PETS三级/公共英语三级【年中特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
Tom always knocks before entering a room.
　　汤姆在走进一个房间前总要敲门。
这句话before entering a room=before he enters a room
这里通过介词before+动名词entering，将一个时间状语从句变成了动名词结构
He apologized for not writing to you.
　　他为没给你写信而表示道歉。
通过介词for+动名词writing，将原来要两个句子来表达的并成了一句
原来两句表达为He didn't write to you. He apologized for that
You can't leave the restaurant without having paid the bill.这句话补充成从句就是You can't leave the restaurant if you have not paid the bill.从句是现在完成时，主句是情态动词，表示如果没有付款，就不能离开餐厅
希望对同学有帮助~