Grammar JLPT Level N4 - Lesson 25

Structure of ～た所(ところ)だ:

Verb［た］⁽¹⁾ + ところ + だ

⁽¹⁾ Verb［ていた］

Details about ～た所(ところ)だ:

In a similar way to how ところ (sometimes shortened to とこ) means 'about to (A)', or 'on the verge of (A)', when following the non-past form of a verb, when following the auxiliary verb た, ところ will mean 'just did (A)'.
To use this structure, make the past tense of any verb, and then add ところ.

今(いま)、先生(せんせい)に聞(き)いたところ。
I just asked the teacher.
仕事(しごと)は今(いま)終(お)わったところ。もうすぐ帰(かえ)る。
I just finished work. So I'll head home soon.

While this expresses 'just did (A)', ていた may be used when you would like to express that you were 'just doing (A)'.

休(やす)みを楽(たの)しんでいたところで上司(じょうし)から電話(でんわ)が来(き)た。
I got a call from my boss just as I was enjoying my day off.

～た所(ところ)だ

Usage Notes for ～た所(ところ)だ:

When the kanji form of 所(ところ) is used, it is often referring to an actual physical location, rather than a situation/standpoint. However, this is something that native speakers will sometimes mistake. Therefore, seeing the kanji form and the hiragana form are both relatively common.