As you might have begun to suspect, there are a lot of ways to express your suppositions and assumptions in Korean. The form -(으)ㄹ걸요 is yet another way to express one of your unconfirmed suppositions or assumptions. This form, however, you only use in spoken language, not written language.
Conjecture And Supposition
-(으)ㄹ걸요 is a form you can use to express your unconfirmed suppositions or assumptions about future events or something you are simply unsure of. In terms of certainty, this form is weaker in conviction than the form -(으)ㄹ 거예요. Also, as mentioned earlier, the form -(으)ㄹ걸요 is a form you use only in spoken language. Furthermore, you use it when talking to close friends. It is not a form used in more formal settings.
Also, there is a restriction to this form. You can only use this form when you have the knowledge to back up your assumption the listener is unaware of. If both you and your conversation partner are aware of the basis of the assumption -(으)ㄹ걸요 cannot be used.
가: 민수 씨를 못 봤어요. 어디에 있는지 혹시 아나요?
나: 아마 아플걸요. 어제 기침을 많이 했어요.
A: I don’t see Minsu. Do you happen to know where he is>?
B: Maybe he is sick. Yesterday he was coughing a lot.
The form -(으)ㄹ걸 comes with an additional layer of complexity in informal speech (반말체). You cannot only use it to express unconfirmed assumptions or suppositions. You can also use it to express your regret over past events. This is possible because it is the contracted form of the form -(으)ㄹ 그랬다. As such you will have to pay close attention to what is being said as it might just be that regret is being expressed rather than an unconfirmed supposition or assumption. Also unlike when expressing suppositions or assumptions, the intonation does not rise when expressing regret.
가: 와…노트북이 30%나 세일을 하네.
나: 진짜? 지난 달에 새로운 노트북을 샀어. 세일할 줄 알았으면 가다릴걸.
A: How is the taste of the kimchi jjigae?
B: Really? I bought a new laptop last month. If I had known it was going to be on sale, I would have waited.
How To Use
Attaching the form -(으)ㄹ걸요 is pretty similar to the intermediate forms -(으)ㄹ 텐데 and -(으)ㄹ 테니까. So, to do it, you need to follow the ‘batchim/no batchim’ rule with the deviation that a verb stem with a ㄹ as batchim are treated as no batchim endings. Of course, you still need to keep an eye out for those Korean irregular verbs. Though by this point they shouldn’t be causing you any problems.
You can use this verb with verbs, adjectives and nouns. So you needn’t worry over word types.