~돼 is usually used for giving or asking permission
examples;
자면 안돼
you shouldn't sleep
너희 집에 가도 돼?
can I go to your place?
아 and 어 are the most common letters used at the end of the sentences when speaking with the informal form
examples;
괜찮아
That's ok
울었어?
Did you cry?
에 usually comes after a place or time
examples;
학교에
To school
밤에?
At night?
10월에?
On October?
서 is for confirming certain information
유학간다면서?
I heard you're studying abroad?
아팠다면서?
I heard you were sick?