The expression plain base+ㄹ 수 있어요 or 없어요 which means can or cannot <verb> also corresponds to the meaning of plain verb expression whether positive or negative. Only, the pattern conveys possiblity and or ability.
The expression ㄹ 수 없어요 (-l su opseoyo) also means the same as the negative expression 못 followed by verb as in the case of 갈 수 없어요 (kal su opseoyo) is the same as 못 가요 (mot kayo); the first expression meaning cannot go and the second one emphatically means cannot go (sure that the person cannot go).
Here are some applications of this verb ending:
1. 내일 후에 시험 집에 갈 수 있어요. (Naeil hue shiheom jipe kal su isseoyo). I can go home tomorrow after the exam.
2. 오늘 아침 도서관에 올 수 있어요? (Oneul achim dosogwane ol su isseoyo?) Can you come to the library this morning?
3. 새 가방이 볼 수 있어요? ( Sae kabangi bol su isseoyo) Can i see the new bag?
4. 동생이 파티에 갈 수 없어요. (Dongsaengi pati.e kal su opseoyo) My younger brother cannot go to the party.
5. 내가 바뻐서, 이메일 답장을 보낼 수 없어요. (Naega papposo, email dapjangi bonael su opseoyo) I cannot send my email reply because I am busy.