This pattern shows a speaker's conjecture, supposition, or intention depending on how it's used.

- is proceded by the past, present, or future modifier suffixes (-ㄴ/은, -는, -ㄹ/을)- when the subject of the sentence it's used in is in the third person, it expresses the speaker's conjecture or supposition based the appearance of a certain fact or event
- when used with the future modifier suffix (-ㄹ/을), it usually indicates a speaker's intention and is usually in the first or second person
— with the future modifier suffix, it normally is used with the first person declarative (stating a fact) or second person interrogative (question) forms

Examples:
비가 올 셈인가?/I wonder if it will rain?
제 친구가 많이 공부한 셈이야./I bet my friend studied a lot.
내일 돌아올 셈입니다./I'm going to return tomorrow.