because of, therefore
-아/어서 is used to express that the information in the first clause is the reason for or the cause of the succeeding clause.
It corresponds to 'because (of)', 'on account of', and 'so... that...' in English.
If the stem ends in the vowel ㅏ or ㅗ, then -아서 is used.
Otherwise, -어서 is used.
As for verbs ending in 하다, the form changes to 해서, and in the case of 이다, 이어서 is used, except in conversation, when 이라서 is used.
-아/어서 cannot be used in imperative or propositive sentences.
(See also "A/V-(으)니까 ① Reasons and Causes": click here)
Tense markers such as -았/었- and -겠- cannot come before -아/어서.
The first clause is the reason for or the cause of the second clause.
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