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?
*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.
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."
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.
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.
I agree with Jim Balter. Past progressive is usually used to describe two actions that happen at the same time.
"When the typhoon came, I was walking on the beach."
"I was blowing out the candles on my birthday cake as my long lost friend walked in."
"When I kissed you, I was mistaking you for my wife."
It still sounds awkward however.
Mistaken is a past participle, and in "I am mistaken," it is used correctly as a predicate adjective. Parallels: I am hurt, I am bundled up, I was seated late, she is well brought up, etc.
In the case of "I am mistaking," the verb "to mistake" is transitive--that is, it requires a direct object [D.O.], much as "to kick" or "to thank" require direct objects. I must mistake something, kick something, thank something/someone. "I am mistaking you [D.O.] for someone else" or "I was mistaking him [D.O.] for his twin brother" are fine (and tense makes no difference),but we need that direct object in there.
Some verbs may be transitive or intransitive: I eat Kit-Kats [D.O.] vs. I eat [no D.O.] at 6:30. Others are only transitive or only intransitive.
There are two forms here. This is very simple. I simply can't read through the answers that are so long.
To be mistaken [about someone or something] Compare that to:
To mistake one thing or person for another. [a transitive verb]
He was mistaken for a famous actor [by the girl].
She mistook him for a famous actor.
They both mean the same thing.