Question

Helen had left her keys in the office, so she had to wait until her husband came back

Helen had left her keys in the office, so she had to wait until her husband came back中为什么so要改为and

Answer 1

Helen是无意把钥匙丢下的，不够成因果关系。and作为并列连词比较好。

追问

为什么不构成因果关系呢，是因为丢了钥匙才等呀，我还是不大明白，能再解释一下吗？