がいいYou should, It would be good to

がいい is a combination of the case-marking particle が, and the い-Adjective いい. It is used in a way that almost sounds like an order from someone in a higher position, to someone in a lower position. Standard translations will include 'you shall (A)', or 'you should (A)'. However, the more literal translation is 'to (A) would be good'.

(A) will be the dictionary form of a verb. がいい will then directly follow (A).

In many cases, the person using がいい is in a very high position, or perceives themselves to be in a very high position such as a king, emperor, wise wizard, or something similar. Due to this, がいい is rarely used in real life, and sounds very magnanimous, or even pompous.

Additionally, as いい is being used in a pompous way, the more literary 良(よ)い will frequently be used in its place.

Fun Fact

Verb + がいい may be thought of as a far stronger equivalent of た方(ほう)がいい 'you should (A)', except that it will only ever be used by those that consider their own suggestion as the only correct course of action.