grammar - What is the difference between 前に and 先に when expressing order of events?

When considering these sentences

1. 食べる前に宿題をする。


2. 食べるより先に宿題をする。

They both mean I will do my homework before I eat.

If I had to guess, the nuance would be for 先に means "I will do homework ahead of eating" and 前に focuses more doing the action of eating before doing the homework. But I'm not so sure about my interpretation.

Can someone explain the difference and in which particular contexts they can only be used with some examples?

Answer

食べる前に宿題をする。
食べるより先に宿題をする。

You're right that they both mean "(I will) do my homework before I eat", both 前に and (より)先に can mean "before" or "earlier" and these can be used interchangeably in most situations where you describe one action occurring before another.

The only difference that I can see in your specific examples is... 先に can have a nuance of order of priority/precedence (優先順位), so I think 食べるより先に宿題をする can have a slight nuance that 宿題 has priority/precedence over 食べる, is more important and should be done earlier than 食べる, whereas 食べる前に宿題をする just neutrally means "to do homework before eating".

And ～する前に can often mean "right before".

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A few examples using 前に or 先に (when used in the sense "before/earlier"):

お先にどうぞ。（× 前に） After you.
先に行くよ。（× 前に） I (will) go/leave first.
何よりも先に　First of all / before anything else
お先に失礼します。（× 前に） I'm going home (before you). → Good-bye. (at the office etc.)
３月３日より前に返事をください。 Give your answer before March 3.
^{(～より先に返事を下さい might sound like you want the answer after March 3.)}
彼より先に着く / 彼の前に着く / 彼より早く着く arrive before/earlier than him
^{(前に can mean "right before".)}