아/어/여서

Used to connect two clauses, the first clause provides the reason or cause for the second clause.

It can not be used to make commands or suggestions.

It can be used at the end of a sentence to give reasoning for something that was just said.

Used to replace 고 when it follows a verb that means "to come" or "to go" (오다, 가다, 들어오다, etc.)

Like 고 it is used in many expressions so it is important to know.