-(으)ㄹ 따름이다

Says that something is simply this and nothing more; it is a formal, often humble way to say 'I merely…' or 'I can only feel…'.

정도·비교Degree & comparison

Han: The speaker was forced to take a taxi, so necessity is needed: -(으)ㄹ 수밖에 없다. -(으)ㄹ 따름이다 means 'merely', not 'had no choice but'.

Examples

저는 제 할 일을 했을 따름입니다.I merely did what I was supposed to do.

이렇게 큰 상을 주셔서 그저 감사할 따름입니다.I can only say thank you for giving me such a great award.

부족한 저를 믿어 주신 분들께 죄송할 따름입니다.I can only apologize to those who trusted someone as inadequate as me.

갑작스러운 소식에 그저 놀라울 따름이었다.At the sudden news, I was simply stunned.

V/A+-(으)ㄹ 따름이다

V/A+-았/었을 따름이다 (past)

N+일 따름이다

감사하다 → 감사할 따름입니다; 하다 → 했을 따름입니다

It is formal and bookish. It is typical of speeches, interviews and writing, and it sounds stiff among friends. In casual talk use -(으)ㄹ 뿐이다 or 그냥 instead.
For a past action, put the tense before 따름: 했을 따름이다 ('I merely did'). 했던 따름이다 and 한 따름이다 are wrong.
It does not express necessity or 'having no other choice'. It does not mean 'can only' in the sense of ability either. It only limits the statement to 'just this'.
It closes a statement and cannot be made into a command or suggestion. It very often follows adjectives of the speaker's feelings, such as 감사하다, 죄송하다, 놀랍다 and 안타깝다, usually with 그저.

Wrong: ~~막차가 끊겨서 택시를 탈 따름이었어요.~~

Right: 막차가 끊겨서 택시를 탈 수밖에 없었어요.

The speaker was forced to take a taxi, so necessity is needed: -(으)ㄹ 수밖에 없다. -(으)ㄹ 따름이다 means 'merely', not 'had no choice but'.

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