grammar - Particles following ところ （に, を, and で）

• 料理をしているところに電話がかかってきて困った。


• デートをしているところを友だちに見られてしまった。


• 電車に乗ったところで今日は祝日だと気づいた。

In the above sentences, what decide the particle following ところ? They all seem to indicate a happening during/while the user is engaged in an activity.

Answer

As user4092 said, it is the verb that matters.

I will break down each sentence by explaining the verb.

1. 
   

   
   

   料理をしているところに電話がかかってきて困った。

   
   

   
   

   電話がかかってくる accepts a number of things marked by に, and one of them is the time (compare: 「仕事中に電話がかかってきた。」).

   
   

   Thus, 料理をしているところ in this first sentence is functioning as a time.

   
   
   


2. 
   

   
   

   デートをしているところを友だちに見られてしまった。

   
   

   
   

   Explaining this one is a little complicated since it's just a complicated sentence (using the adversarial passive), so you may want to come back to this after you know more Japanese, but anyways...

   
   

   _{Necessary tangent: 見られる is the passive of 見る. When you form a passive, you lift something from the underlying sentence to が, and the が-marked thing in the underlying sentence to に.}

   
   

   _{In this case, the original sentence is 友達が［（私が）デートをしているところ］を見た, and the 私が gets promoted to the が-marked thing of the passive, and of course 友達 becomes the に-marked thing:}

   
   

   
   

   <sub>友達が　　　［（私が）デートをしているところ］を　　　　　　　　　見た
   
   ⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓
   （私が）　　　　　　［デートをしているところ］を　　友達に　　見られた</sub>

   
   

   
   

   So, the thing being marked by を is the thing being seen.

   
   

   As such, デートをしているところ here is not behaving like a time as in (1), but instead as a noun referring to a past event.

   
   


3. 
   

   
   

   電車に乗ったところで今日は祝日だと気づいた。

   
   

   
   
   

   Unlike (1), 気付く doesn't accept times in the に position.

   
   

   So, instead, 電車に乗ったところ here functions as a "location in time" where you did the realizing of 今日は祝日だ.

   
   

   Compare, 「電車に乗った時点で今日は祝日だと気づいた。」, where 〜時点 is another "location in time" sort of thing.

   
   

Basically, the complicated thing here is not ところ -- ところ is just flexible in terms of what it can behave as (time, location, noun) -- and that flexibility brings out the complexity of verbs in Japanese.

Being able to pick the right place to jam ところ with a verb more or less comes down having a good feeling for the verb and understanding what arguments it accepts.