"-ㄹ/을걸요", "-ㄹ/을 것이다", "겠다" and "-ㄹ/을 것 같다" They are all related to guessing about something, but also different in their usage. * -ㄹ/을걸(요) - I'm pretty sure ..." (often contradicts what the other person said). Only used in speech. * -ㄹ/을 것이다 - it will be ...; it's likely that ... (expresses a guess or prediction). More literary than -ㄹ/을걸. * -ㄹ/을 것 같다 - it appears that ... (a cautious guess or interpretation. 같다 makes it weaker than (것)이다) * -겠다 - 1) (for 1st person) I might as well ..., I will ..., 2) (2nd and 3rd person) you/they might ... (possibility). For example, about a guy and girl who appear romantically involved, their friends might say: 1. 걔네들 서로 좋아할걸? - They are in love, don't you think? 2. 걔네들 서로 좋아할 거야 - I think they are in love / I think they will fall in love. 3. 걔네들 서로 좋아할 거 같다 - It appears they'll be in love. 4. 걔네들 서로 좋아하겠다 - They might fall in love. All these forms except -겠다 can take the -ㄴ/은/는 ending too, which expresses similar meanings about what is happening now as opposed to what might or will happen.