막상 + -(으)니까 / -(으)려고 하니
when it actually comes to it, actually (in reality)
Example
막상 시험을 보려고 하니 너무 떨렸어요.
When it actually came time to take the test, I was so nervous.
Good to know
Contrasts expectation with reality: 막상 해 보니 쉬웠어요 'once I actually tried, it was easy'.