〜るところだ

Indicates that someone is just about to do something: the action is imminent but has not yet started. It describes the moment right before an action begins.

Neutral in register and common in everyday speech, often with 今 or ちょうど to stress that the action is about to begin. It is typically used to explain one's current situation, for example when someone calls or asks what you are doing. It contrasts with 〜ているところだ (in the middle of an action) and 〜たところだ (just finished).

• 〜ているところだN3「〜ているところだ」 attaches to the verb's て form + いる and shows the action has already started and is in progress; it is also often used to explain that you are busy and cannot attend to something else. 「〜るところだ」 attaches to the dictionary form and means the action has not yet started, so the two cannot be swapped.
• 〜たところだN3「〜たところだ」 attaches to the verb's た form and means the action has just been completed; it cannot be used with time words from long ago like 「去年」. 「〜るところだ」 means the action has not yet started, so the two are at opposite stages and cannot be interchanged.
• ところだったN2「ところだった」 can serve as the past tense of this pattern, meaning 'was just about to do'; it can also mean 'almost happened (but did not)', attaching to the ない form with a tone of relief or a shudder. This second meaning is absent from 「〜るところだ」, so context is needed to tell them apart.
• ようとするN3「ようとする」 attaches to the verb's volitional form and is close to this pattern only in the 'about to' sense, often describing the instant an action begins or natural phenomena, and is common in written description. It also has a 'try/attempt to do' sense that 「〜るところだ」 lacks.
• ところN4「ところ」 covers all three stages of an action: with the dictionary form it is exactly this pattern and the two are interchangeable; with the 「ている」 form it means in progress, with the た form it means just finished, and it can also work adverbially as 「ところに／ところへ」.
• ようと思うN4「ようと思う」 attaches to the volitional form and expresses the speaker's intention, and can also describe long-term plans; with 「ようと思っている」 it can even describe someone else's intentions. 「〜るところだ」 takes the dictionary form and only means the action is about to begin, so the two generally cannot be interchanged.

〜るところだ means 'about to do', unlike 〜ているところだ ('in the middle of doing') and 〜たところだ ('have just done'); ところだった shifts this to the past, meaning either 'was just about to do' or 'nearly happened (but didn't)', depending on context. ようとする stresses trying or being on the verge of acting, ようと思う expresses intention rather than imminence, and ところ on its own is the general noun for a place or point.