1(temporal) An adverb meaning previously, earlier, or beforehand — referring to a point in time before the present. Often used to refer back to something previously said or written, as in 先に話したように ('as I mentioned earlier').
先に申し上げたとおり、会議は来週に延期になりました。
As I mentioned earlier, the meeting has been postponed to next week.
その店には先に一度行ったことがあります。
I've been to that shop once before.
先に紹介した方法のほうが効率的だと思う。
I think the method I introduced earlier is more efficient.
2(order) An adverb meaning first, ahead of (someone else), or before others. Extremely common in everyday set phrases like お先に失礼します ('excuse me for leaving first') and お先にどうぞ ('after you / please go ahead'). The pattern 〜より先に ('earlier than 〜') is also basic vocabulary.
お先に失礼します。
Excuse me for leaving first / going ahead. (used when leaving the office before colleagues)
お先にどうぞ。
Please, go ahead. (yielding to someone)
彼は私より先に駅に着いていた。
He had arrived at the station before me.
子どもたちが先にご飯を食べ始めた。
The children started eating first.
3(preparation) An adverb meaning beforehand, in advance, ahead of time — used when completing something before some other action or event. The typical pattern is 先に〜しておく ('do 〜 in advance').
会議の資料は先に配っておきます。
I'll hand out the meeting materials in advance.
旅行のチケットは先に買っておいたほうが安い。
Travel tickets are cheaper if you buy them ahead of time.
忙しくなる前に、先に宿題を終わらせておこう。
Let me finish my homework first, before things get busy.
出かける前に、先に電気を消してください。
Before going out, please turn off the lights first.