OK this is probably something between a comment and an answer. I'm probably going to get something wrong. So from the native speakers please point out how I'm getting this wrong.
That said, I too struggled for a long time understanding 先. The word seemed contradictory to me. And, if I look it up in one of my dictionaries (角川国語辞典) I see the following definitions:
- 前の部分。先頭。
- 進んでいく方。先方。
- 前途。将来。
- はし。末。
- 過去。以前。
- 相手。先方。
- 目的の所。 到着する場所。
- より早く位置をとること。
- 先物。
It would seem that definitions 3 and 5 contradict one another. But, I think that's only if you try to take this word a bit too literally. Generally, context should point out the desired meaning. And, it's possible something could be translated into English in two different ways depending on how one thinks about things.
Yet overall, for myself, what I've found helpful with this seemingly confounding word is to think of it along of the lines of that which lies in front/ahead.
When you're talking about events that haven't yet happened, then they still lie in front of us. Hence they are in the future. This would be like your example sentence
先の事など誰にも予想できない
You don't predict what already happened; you predict what has yet to come. So what lies ahead is the future.
It's much the same with
卒業式は二ヶ月先だ.
Then there are set phrases like 先週 and 先日 where it's perhaps just best to memorize them and to try to avoid overanalyzing them.
With expressions like
５年先を見る
and
５年先に生まれた
I would take two things into consideration: (1) the form of the verb and (2) the logic of the sentence.
- If the verb is non-past then 先 most likely refers to something yet to come (something that lies ahead of us).
- If the verb is past, then most likely you're talking about something that preceded the moment now.
Regardless of what the particular meaning of 先 might be, it's a relative term, relative to a particular point of view that needs to be discerned. So that's what needs to be teased out: what's the perspective of this sentence.
As I've already mentioned above, it's also perhaps useful to learn certain fixed expressions and then be able to extrapolate from them. For example,
お先に
means, After you! not me first!
Shift the context slightly and
お先に失礼したいのですが
which means I'd like to leave before you (well, really it's an announcement, not a request). Getting hung up on the before/after aspect here is to miss the point. お先に refers to the person being spoken to; in a manner they take precedence.
I would think the best thing to do is to go old-school and open up one of those relics of the past, a printed and bound Japanese-English dictionary. They're organized very well. They often provide a wealth of example sentences under headers for the different senses of a particular entry. Memorizing these for words like 先 can help build your intuition about how to discern the meaning and contexts.
Regarding on-line or electronic dictionaries, they tend not to clearly organize example sentences (if at all) in a manner that facilitates teasing out nuances. (If someone knows of a good on-line dictionary that does so, please rely that information in the comments.) For example, Jisho.org provides sentences but not nicely organized by subheadings or different senses of the same word. My Apple Japanese Dictionary does a bit better job, but I still find my print version has much better examples.
