语法总结
used to / would 表示过去反复动作
本课里 used to sit 和 would bark 都用来写过去经常发生的习惯性动作。used to 更适合交代过去状态，would 更适合放进叙事循环里。
相似句：
- Rex used to sit outside the gate every evening.
- Whenever he saw me, he would run to the door and bark.
目的表达：to do / so that ... would ...
train him to press... 用不定式表达目的；he was barking so that someone would let him out 则用 so that 从句把目的写得更完整。
相似句：
- My husband trained him to open the gate with his paw.
- He barked so that someone would come and help him.
时间链：every time / as soon as / until
本课的笑点建立在动作循环上，所以大量使用时间连词串起“每次想进来 - 一开门 - 一直等到关上”的过程。
相似句：
- Every time the bell rang, the dog rushed to the gate.
- As soon as the door shut, he began barking again until someone opened it.
so...that... 的省略现象
Rex got so annoyed we have not seen him since. 这里口语化地省略了 that，完整表达应是 so annoyed that...。
相似句：
- He was so tired that he fell asleep at once.
- The child was so excited that he could not keep still.