Example sentences
bìng病le了，suǒ所yǐ以méi没lái来。
Sick, so didn't come.
suǒ所yǐ以ne呢？
So what?
About this word
所 is a nominalizer, roughly "that which", and 以 means by means of. In classical Chinese 所以 meant "that by which", i.e. the reason or the means, which is why 所以然 still means the how and why of a matter. The modern word has narrowed to therefore, and it lives inside the frame 因为...所以... Either half can be left out, and 因为 without 所以 is common, but opening a sentence with 所以 alone sounds casual: 所以我迟到了.
The story behind the character
所
所 is 戶 (a door) with 斤 (an axe) beside it. Xu Shen defined it as 伐木聲也, the sound of chopping wood, and quoted a line of old poetry, 伐木所所, to prove it. So 所 was an axe-noise character. It got borrowed to mean "place" and "that which," and the chopping sound is long gone.
以
以 in the oracle bone is a person carrying something, or a tool in use. Xu Shen wrote 以, 用也, to use, and explained the shape as a reversed 巳. That one meaning explains a lot of grammar: 可以, can use it, 所以, that by which. It is one of the busiest characters in the language, and underneath it is still a hand carrying something.