Grammar
그래야
only then / that is what is needed
Form
그래야: “that is what needs to be done”, “only then”. A set form based on 그러하다/그러다 (“be/do so”) plus -아/어야, marking a necessary condition.
Function
Refers back to an already mentioned action or situation and presents it as a necessary condition. It may be followed by 하다, 되다, or a clause describing an outcome.
Why this form?
그렇게 해야 하다 — “must do it that way”: explicitly expresses an obligation with 하다; 그래야 can introduce the condition needed for a result.
How to build it
그래야 하다 / 그래야 되다: need to do so; 그래야 + clause: only then can a result follow. Not all such uses impose an obligation on the listener.
Forms not to confuse
그렇게 해야 하다
그렇게 해야 해요. — You have to do it that way.
Examples
저도 그래야 할 것 같아요. — I think we should do that too.
그래야 문제를 해결할 수 있어요. — Only then can we solve the problem.