ところだった

1. 1Something (usually bad) came very close to happening but was avoided: "almost / nearly did ~".Ex. 1
2. 2With adverbs like 危うく, it stresses a narrow escape from danger or trouble.Ex. 2
3. 3Describes being on the point of doing something at a past moment: "was just about to ~". It does not say whether the action then took place.

Neutral in register and used in both everyday speech and writing, often to look back on a close call with relief. Unlike 〜るところだ, which describes the present point just before an action, ところだった refers to a past point: either a close call that was avoided, or simply the moment just before an action.

• 〜るところだN3「〜るところだ」 is anchored to the present or to some reference point and means one is about to do something, with no sense of "almost." Sense 2 of 「ところだった」 is simply its past tense with the same meaning, and to say something "nearly happened" only 「ところだった」 can be used.
• もう少しでN2「もう少しで」 is an adverbial phrase placed before the verb that often pairs with 「ところだった」 to convey "almost"; on its own with the 辞書形 it means "on the verge of...", and with 「可能形た形＋のに」 it means "almost managed to... but didn't." The two form a set combination and are not interchangeable.
• ところN4「ところ」 marks only the point in time when an action is about to happen, is happening, or has just been completed; it attaches to the 辞書形, ている形, or た形 and carries no "almost" nuance. For "was about to do at the time," 「ところだった」 is the past tense of 「辞書形＋ところだ」.
• 〜たところだN3「〜たところだ」 attaches to a verb's た形 and means the action has just been completed, while 「ところだった」 attaches to the 辞書形 and means something almost happened but didn't, or was about to happen at the time. They are not interchangeable: 「落ちたところだった」 means "had just fallen at that moment."
• 〜ているところだN3「〜ているところだ」 attaches to 「て形＋いる」 and means the action is in progress at the moment of speaking, whereas 「ところだった」 attaches to the 辞書形 and means something almost happened or was about to happen — the action has not yet started. They sit at different stages and cannot be swapped.

〜るところだ means "about to do" now, 〜たところだ means "have just done", and 〜ているところだ means "in the middle of doing". ところだった places the "about to" point in the past and often marks a close call, especially with the adverb もう少しで ("almost"). The noun ところ ("place; point, moment") underlies all of these patterns.