所以 therefore, so ,as a result
Luchang duche, SUOYI wo chidao le.
There was a lot of traffic, so I am late.
Ta zhi SUOYI chenggon, shi yinwei jianchibuxie.
The reason for her success, is her unremitting effort.
As you see they have both almost the same sense ,but they won't be always interchangeable ,each of them will be suitable for a certain sentence strucutre and context.