Question for a sentence in Adelphoe

Question

In Terence's Adelphoe 3.4, there is:

fient quae fieri aequum est omnia.

omnia, quae, fient are all plural, why is aequum est singular, what is the role of quae in its clause?

Answer 1

Aequum est means “it is right, proper” and usually takes an AcI to say what is right and proper. For example, aequum est vos ignoscere means: “It is proper that you should forgive.” Passive infinitives are, of course, also welcome.

In this case, the AcI is quae fieri, “which to be done.” This is part of the relative clause quae fieri aequum est, “which is proper to be done.” Thus the meaning of the whole sentence is:

All that is proper to be done, will be done.

(The neutral plural, as often, is used to talk about generic “things” without specifying them further.)