ところ

1. 1Be just about to do something (dictionary form + ところ).Ex. 1
2. 2Be in the middle of doing something (ている + ところ).Ex. 2
3. 3Have just done something (た-form + ところ).

Used in everyday speech and writing to describe exactly where an action stands in time, often to explain one's current situation (今、出かけるところです). With に/へ, it sets up a moment into which another event breaks, such as a phone ringing. The form of the verb before ところ decides the stage: before, during, or right after.

• 〜るところだN3「〜るところだ」 corresponds only to sense 1 (dictionary form + ところ), matching it in both meaning and conjugation, so the two are interchangeable, but it does not express an action in progress or just completed. Also, 「るところだった」 can additionally mean something almost happened.
• ているところN4「ているところ」 is exactly the sense of this pattern with the ている form, expressing an action in progress, and the two are interchangeable in this sense; it cannot be used for being about to do something or having just finished. It also does not attach to stative verbs such as 「ある」 or 「知る」.
• たところN4「たところだ」 is the usage of 「ところ」 with the た form, expressing just having finished, and the two are interchangeable; 「たところ、…」, however, expresses discovering a result after doing something, does not require the action to be just completed, can be used with 「先月」, and is usually followed by a past-tense clause.
• たばかりN4「たばかり」 is similar only to 「たところ」: 「たところ」 refers to the very moment of just finishing and is relatively objective, while 「たばかり」 is based on subjective feeling and can span a longer time, so 「先月来たばかり」 cannot be replaced with 「たところ」.
• 〜たところだN3「〜たところだ」 is the usage of 「ところ」 with the verb's た form and expresses only just having finished; 「ところ」 can also take the dictionary form or ている form to express being about to happen or in progress, and can be used adverbially as 「ところに／ところへ」.
• 〜ているところだN3「〜ているところだ」 is the sense of 「ところ」 with the ている form and expresses only an action in progress; 「ところ」 can also take the dictionary form for being about to happen and the た form for just having finished, and can be used adverbially as 「ところに／ところへ」.

〜るところだ, ているところ/〜ているところだ, and たところ/〜たところだ are the specific 'about to', 'in the middle of', and 'just did' uses of this ところ. たばかり also means 'just did', but it can describe an action felt as recent even after some time has passed, while たところ means right at that moment.