he was barking so that someone would let him out

问题描述:

he was barking so that someone would let him out
he was barking until someone let him out.为什么这两句.一个后面要加would一个后面不加?

两句意思不同
第一句:他一直大叫,希望有人会把他放出去(过去将来时)
so that ...以至于 引导结果状语从句
后面“把他放出去”的动作是他希望的结果,还没有发生,是过去将来时
第二句:他一直大叫,直到有人把他放出去(过去时)
until 直到 直到这个动作发生,前面主句的动作停止 引导时间状语从句
后面“把他放出去”的动作发生了,是一般过去时