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I've always been curious why some people insist that "I was mistaken" is grammatically correct whereas "I was mistaking" is grammatically wrong. Doesn't the later follow past progressive verb tense? Can someone give me an example where each is right or wrong?

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*I was mistaking

This phrase is not a complete sentence. You would have to add what you were mistaking.

I was mistaking you for a stranger, but now I found out you have been living here for three years.

Now for mistaken, it is used as an adjective, in the same way (and similar in meaning) as you would use wrong:

I was mistaken, you are not a stranger. I was wrong, you are not a stranger.

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I'm not even certain I was mistaking you for a stranger is the correct use of the tense; I would generally expect I had mistaken you for a stranger. Generally, to mistake something takes only an instant, so even if someone still mistakes you for someone else, you would say I think you've mistaken me for somebody else. It is very rare to refer to the instant of the mistake, but perhaps I forgive you for mistaking me for somebody else. would work. – Magus Jan 28 '14 at 19:43
I mistook you for a stranger. – Jim Balter Jan 28 '14 at 21:20
" the correct use of the tense" -- It can be: When I said "The man climbing over my fence is a stranger", I was mistaking you for a stranger. – Jim Balter Jan 28 '14 at 21:58
How is "I was mistaking" an incomplete sentence, when "I was running" is not? to run and to mistake are each intransitive verbs. Certainly "mistaking" sounds odd, but that doesn't make it incorrect. – Phil Frost Jan 28 '14 at 22:23
@PhilFrost: to mistake is intransitive? Can you please give an example of it being so used? I can imagine I am running, I am eating, but surely not I am mistaking. "So what do you do when you get home? - I read, I eat, I mistake for a while and then I sleep"?? – oerkelens Jan 28 '14 at 22:29

In this case "mistaken" is not being used as a verb, but an adjective. In "the tall brown mistaken horse", "tall", "brown" and "mistaken" are all adjectives.

"Mistaking" can make sense, but would usually be used when you're mentioning a particular mistake you were making. "I was mistaking you for somebody that cares." Or if you'd like to refer to a single mistake rather than a habitual one, "I mistook you for somebody that cares." Or, sticking with mistaken, "I was mistaken in thinking you were somebody that cares."

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I have a slightly different resolution to this conflict from the other answers, although I agree with them as well.

There are two related verb phrases:

(1) "to mistake [object] for [second object]" [meaning: to erroneously think that object 1 is object 2]."

and the more common

(2) "to make a mistake." [meaning: to err.]

The reason you cannot say "I was mistaking" is because the verb "to mistake" as in (1) is transitive. (In fact, semantically it must take two objects, separated by the word "for.")

The phrase "I was mistaken" refers always and only to the second of these. If you wanted to use the past progressive, you could certainly say "I was making a mistake." (This is exactly taking the verb in (2) and turning it into a past progressive form, which is what you wanted to do with the verb in (1), but which you were told not to do). It means more or less the same thing as "I was mistaken," but emphasizes the process more, as is what you would expect from the past progressive.

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Both constructions are grammatical, but only one is also idiomatic.

With the meaning "I was wrong", only "I was mistaken" is used. "I was mistaking" could be easily encountered in a different context, though: "I was mistaking X for Y". There, in turn, "I was mistaken" wouldn't be possible.

There is actually a name for this linguistic phenomenon: blocking. When two theoretically equally valid ways of saying something conflict, the existence of one blocks the other from getting any traction.

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"I was mistaking" is not a complete sentence, any more than "I was replacing". As tobyink notes, "mistaken" is an adjective, not a verb. – Jim Balter Jan 28 '14 at 21:18
@Jim I never said it was a complete sentence, and neither did the OP. I am answering the question at face value. (And by the way, just because a sentence is nonsensical, doesn't make it incomplete. But again, that's beside the point here.) I also never said "mistaken" was a verb. Edit: In fact, my answer is all but identical to tobyink's and oerkelens'. – ЯegDwight Jan 28 '14 at 21:26
I never said it was a complete sentence, and neither did the OP. -- Irrelevant strawman; the OP offered it as a complete sentence, as a substitute for "I was mistaken". As such, it's ungrammatical, and certainly not "theoretically equally valid". I also never said "mistaken" was a verb. -- Irrelevant strawman. I didn't say you said it, I said it. I said it because it's relevant, as the OP asked about verb tense. my answer is all but identical to tobyink's and oerkelens -- this is flat out obviously not true. – Jim Balter Jan 28 '14 at 23:31

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