Difference between 「際」 and 「時」?


Both seem to be translated as "when". Are they the same? It seems to me that
「時」 is mostly used for things that actually happened in the past whereas 「際」
is used for when something (could) happen in the future.




> Both seem to be translated as when. Are they the same?

According to synonym dictionaries you can find 折 and 場合 besides 際 as synonyms
of 時.

特別な出来事のあるとき _when there is a special event_ ： 時｛とき｝・際｛さい｝・場合｛ばあい｝・折｛おり｝

その時点を指す表現 _expression that points to that moment or point in/of time_ ：
際｛さい｝に、時｛とき｝に、の折｛おり｝、隙｛すき｝に  
I don't want to let 隙に be included in this group.

> It seems to me that 「時」is mostly used for things that actually happened in
> the past whereas 「際」is used for when something (could) happen in the future.

No, not exactly. You can say both (A) and (B) as natural Japanese.

  * (A) 明日｛あした｝来る **時** これ持って来てね。 _Bring this when you come tomorrow._
  * (B) 昨日｛きのう｝会った **時** 帽子｛ぼうし｝をかぶっていましたね。 _When I saw you yesterday you were wearing a hat, right?_

時 and 際 are not always intechangeable. (C) and (D) are unnatural.  
(E) and (F) are better than (C) and (D), but they are still unnatural.

  * (C) 明日｛あした｝来る **際** これ持って来てね。 
  * (D) 昨日｛きのう｝会った **際** 帽子｛ぼうし｝をかぶっていましたね。 
  * (E) 明日｛あした｝来る **際に** これ持って来てね。 
  * (F) 昨日｛きのう｝会った **際に** 帽子｛ぼうし｝をかぶっていましたね。

I think that 際 is used as a relatively formal or polite expression.  
In (G) and (I), we put a pause after "、".

  * (G) 着席される **際** 、足元にお気をつけください。
  * (H) 着席される **際に** 足元にお気をつけください。
  * (I) 着席される **際には** 、足元にお気をつけください。

