Using the present tense for future events indicates certainty, consistency, and familiarity. In other words, use this to talk about events which will happen, which happen on a regular basis (or are predictable in some way), and about which you have some personal knowledge.
The holidays start next week.
I know this happens every year, last year I saw it happen in the same way, and I am sure it will happen the same way this year.
The train leaves in five minutes
I know the train leaves at a specific time, I know that trains usually leave on time, and I have seen the schedule to confirm when the train will leave.
The gala event starts tomorrow
I have personal knowledge that it is scheduled for tomorrow, I know that events like this usually start on schedule, and I feel comfortable asserting that it will definitely happen.
If any of these three does not exist, there is uncertainty, and the future tense will be more appropriate.
We get paid on Friday
I know this happens with some consistency and regularity, as I've already received at least one paycheck on Friday.
We will get paid on Friday
Oops, now there is some doubt. I hope this will happen, but I'm not sure it'll happen.
Here's another example: given no other context, consider these two sentences:
He will make the winning shot of the game in twenty seconds.
He makes the winning shot of the game in twenty seconds.
The first indicates a future condition, but the context is unclear. Do I really know this will happen? Probably not. More likely I'm expressing an opinion, or a personal hope.
But the second sentence, that expresses an odd certainty. The context is either that we are watching a recording of the game which I have seen before, or that I'm psychic, and I've already seen this happen with my mental powers.
The OP believes that It would be nice to see you before I left expresses a wish and not a real situation, and this is why the past tense is better in the subordinate clause.
It is certainly true that wishes that a present situation were different are commonly expressed in the past tense:
I wish I had more money.
It is also true that the past tense is typically used in the subordinate clauses of similar (so-called) conditional 2 sentences to express an unreal (counterfactual) situation:
It would be nice if I had more money.
The past tense is also used in conditional 2 sentences to convey the speaker's belief in the remoteness of the possibility of something happening:
It would be nice if I finally won some money on the lottery.
Further, the past tense is common (but far from mandatory) in the subordinate clauses of reported statements:
She said that it would be nice to see you before you left.
But none of the above contexts fits the OP's example. The It would be nice to see you clause in this context does not mandate the past tense of the verb in the subordinate clause.
The leaving is prearranged, hence the present tense is used. There is nothing unreal or counterfactual or reported about the leaving. The present tense is the usual tense for scheduled events: cf. My plane leaves at 8.45 tomorrow evening. So, my clear preference in this context is for:
It would be nice to see you before I leave.
Two further points. Firstly, the main clause can be expanded to: It would be nice if I could see you .. , in which case the past tense (could) is used to express a wish, remote possibility or counterfactual. But this still has no influence of the tense in the before I leave clause.
Secondly, the past tense is often used to express deference or politeness. For example:
Excuse me, I wanted to see the manager. Is she free?
So, It would be nice to see you before I left is conceivable in the context of, for example, a student requesting an appointment with a professor.
Addendum
The list below shows the Google results for various "It would be [ nice / good / great ] to [verb] ... before I ... " constructions. They show that the present tense is much more usual in the before clauses of such constructions.
- It would be nice / good / great to see you before I leave [14]
- It would be nice / good / great to see you before I left [0]
- It would be nice / good / great to see you before I go [10]
- It would be nice / good / great to see you before I went [1]
- It would be nice / good / great to know before I leave [4]
- It would be nice / good / great to know before I left [1]
- It would be nice / good / great to know before I go [40]
- It would be nice / good / great to know before I went [0]
Best Answer
The sentence, "I would leave tomorrow", is grammatical, but is lacking some sense (subjunctive mood)
Source: http://englishplus.com/grammar/00000031.htm
"If I would leave tomorrow..." (correct)
ALSO:
"I am going to leave tomorrow." (correct, the day before leaving) "I left [yesterday]." (arrival)
"I will be leaving tomorrow." (correct, the day before leaving) "I left [yesterday]." (arrival)
I will not discuss the use of past progressive here, but I'll provide a link http://www.englishpage.com/verbpage/pastcontinuous.html
You cannot say, "I ate tomorrow" in the same way that you cannot say "I will eat yesterday".
per definition:
Since 'tomorrow' is a future time reference, it is wrong to use it in the above quoted sentences. The point here is you cannot simply use a future time reference in a sentence referring to a past action. Also, it is ungrammatical to say "I would leave tomorrow" (in the context the OP provided). 'Will' is a modal used to indicate future tense, hence the main verb's tense should be changed. As above explained, you may use it if you are indicating a subjunctive mood.
Remember that leave/leaves (present); left (PAST); and will leave(future).