Please follow the reasoning.

Active voice:

My neighbors (always) mistake me for my brother.
That's why they call me Jim.

Passive voice:

I am mistaken for my brother (by my neighbors).
That's why I'm called Jim.


If I'm not mistaken, I wouldn't be called Jim.

But there is an idiomatic expression:

If I'm not mistaken, his name is John.

So if my reasoning is correct, to be mistaken has two meanings:

  • to be mistaken for something/someone (verb)
  • to be wrong (adjective)

Maybe that's why this expression confuses so many English learners.

Note: I've read many related topics on different sites including this one. And I couldn't find a proper answer yet. This Q can hardly be considered a duplicate as the other related Qs are about mistaking vs mistaken.

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    You're right -- if you try hard enough you can make the sentence ambiguous. – Hot Licks Dec 6 '16 at 3:14
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    I don't think that you can remove the "for s.b." part. You need the object, otherwise the meaning changes. – Laurel Dec 6 '16 at 3:35
  • @Laurel According to www.merriam-webster.com the verb mistake not only has the transitive form without a second object: mistook her way in the dark but has the intransitive form too: you mistook when you thought I laughed at you. See goo.gl/iypKmP – Markus Marvell Dec 6 '16 at 3:43
  • @MarkusMarvell But they have different meanings... – Laurel Dec 6 '16 at 3:46
  • @Laurel Yes, you are right. But it does not affect the issue. I can give another example which seems doesn't change meaning: They (usually) mistake me when i'm chewing. I'm mistaken (by them) when i'm chewing. If i'm not mistaken I'd have more friends. – Markus Marvell Dec 6 '16 at 4:08

The confusion arises from the vagaries of English vocabulary and from two similar syntactic forms. Forms first.

  1. Subject Copulative-verb Nominative-Complement

A copulative (or linking) verb is a verb of being or feeling, and it links the complement to the subject, either identifying the subject (if the complement is a noun) or describing the state of the subject (if the complement is an adjective).

[1a] I am John.
[1b] I am happy.

In 1a, John and I are the same person; in 1b, I am filled with happiness.

  1. Subject ( Auxiliary-Verb Past-Participle ) [ by Agent ]

This is the form of a sentence in the passive voice. I've placed the auxiliary verb and past participle in bold parentheses to indicate that they go together to form the predicate. In the passive voice, the subject is acted upon by the verb, and the actor is given as the object of a prepositional phrase with by. I've called the actor Agent, and enclosed the prepositional phrase in bold brackets to indicate that it may be missing.

[2a] I ( am wounded ) by an arrow.
[2b] The President ( is elected ) in years divisible by four.

In 2a, the subject has been hurtfully acted upon by a weapon. In 2b, the subject receives the action of selection for office, but the selectors are not mentioned. We may assume it's voters, and we could supply the missing phrase by the voters.

Confusion may arise because past participles, in addition to signaling the passive voice, may also act as noun modifiers like adjectives. Consider

[3a] I am depressed.
[3b] I ( am depressed ) by events.

3a is like 1b. 3a says that I am suffering from a disease called depression. (I am filled with sad feelings that may have no external agent.)

3b is like 2b. 3b says that external events have acted upon me to make me sad.

But when no agent appears, how do we know which of the templates 1b and 2b to choose? We have to rely on the context. Sometimes the topic makes it obvious, e.g., elections always have agents (called electors or voters) who do the electing. Sometimes it takes knowing English usage and idiom, and that's what it takes for your example.

The OED tells us that the verb to mistake is an old one, with almost 700 years in the language, and originally it had two meanings -- to misidentify and to make an error. But over time, some forms fell out of favor and were no longer used. We no longer say I mistake to mean I made an error. That form is reserved to mean I misidentified as in your example. We say instead I made a mistake for being in the wrong.

The past participle mistaken moved in the opposite direction, and it no longer means I am misidentified unless it's accompanied by a prepositional phrase with for, again as in your example. Alone, it means I am in error.

Thus, if you find am mistaken by itself, it cannot be a passive form of the verb meaning was misidentified, and you must analyze the sentence by the pattern of 1b. On the other hand, if you find am mistaken with a prepositional phrase with for, it must be a passive form, and you must analyze the sentence by the pattern of 2a or 2b, depending on whether the agent is identified in a prepositional phrase with by.

  • Lucidly. I should admit you are right about to be in error and to misidentify. But the verb mistake seems has one more meaning: to misinterpret (goo.gl/iypKmP). And this one still confuses me. Since I could say: My readers always mistake me. That's why they write those angry comments. So i tell my students, If the author is not mistaken (by readers) they have higher ratings. The last sentence seems can be understood both in meaning If the author is right they have higher ratings and also in meaning When the author is interpreted correctly they have higher ratings. – Markus Marvell Dec 6 '16 at 8:46
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    @MarkusMarvell You're right about to mistake having the meaning to misinterpret, and according to the OED, the participle mistaken was used in a straightforward transformation to mean misinterpreted. But that's now archaic. – deadrat Dec 6 '16 at 16:57

It depends on the context

When you say, "if I'm not mistaken..." I am going to conclude you mean "assuming I am not in error." This expression is very common in the native English vernacular of the US and likely other areas as well.

When taking about identities, to be understood I would avoid using the phrase "if I'm not mistaken." Instead, I would replace words as necessary to avoid confusion. Often replacing"if" with "when" will be sufficient.

"When I'm not mistaken for Tom people don't treat me like a jerk."

The "for Tom" is an additional contextual clue that the meaning is different.

Seldom if ever will there be cause for a native speaker of American English to be confused by this expression.

  • To mistake has also the meaning to misinterpret. Example: Don't mistake me, I mean exactly what I said (from goo.gl/iypKmP). So in case My readers always mistake me. That's why they write those angry comments. We can produce the following passive voice sentence: If I'm not mistaken (by readers) I have a higher ratings`. This is exactly the case where the meaning of If I'm not mistaken seems confusingly ambiguous. Isn't it? Though of course it can be fixed by replacing if with when as you've noticed. – Markus Marvell Dec 6 '16 at 8:20
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    "If I'm not mistaken" = "If I'm not in error" in British English, too. – David Richerby Dec 6 '16 at 10:29
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    @MarkusMarvell That would be unidiomatic. "If I'm not mistaken, I have higher ratings" means "I believe I have higher ratings"; if you wanted to express the idea that you have higher ratings unless your readers misunderstand, you'd probably say "If my readers don't mistake me, I have higher ratings." Actually, I'd probably say "misunderstand" rather than "mistake" in that case -- "mistake" in that sense seems a little old-fashioned (but that might just be me). – David Richerby Dec 6 '16 at 10:30

yes it is ambiguous. it can mean either "in error" ("if I'm not mistaken") or "wrongly taken", i.e. "I'm often mistaken for my brother.

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    I disagree. "Mistaken" means "in error"; "mistaken for" means "wrongly taken". – David Richerby Dec 6 '16 at 10:25
  • @David Richerby: "He's been mistaken for years about that." – user175542 Dec 28 '16 at 6:41
  • That's not an example of "mistaken for"; it's just a sentence in which the word "mistaken" happens to be followed by "for". The "for" goes with "years", not "mistaken", as you can see either by rephrasing as "He's been mistaken about that for years" (which, IMO, is a more natural phrasing), by trying to delete "years" (leading to "He's been mistaken for about that", which is nonsense) or by replacing the adjective "mistaken" with something else (e.g., "He's been angry for years about that" -- there's no such thing as "angry for"). – David Richerby Dec 28 '16 at 18:14
  • "mistaken for" is not a word. "He was mistaken for." So "for" goes with the following word: "He was mistaken [for Clark Gable]", not "He was [mistaken for] Clark Gable". – user175542 Dec 28 '16 at 22:18
  • "For Clark Gable" is also not a word... To "mistake for" is a phrasal verb. So, actually, the grammatical unit is "mistaken for". – David Richerby Dec 28 '16 at 22:28

The phrase "to be mistaken" not ambiguous because in English, just as in all other language, you may find context clues when speaking or reading. You are referring to the same arrangement of words that convey different meanings, but they are different versions, if you will, of the word mistaken. Mistaken can mean to be viewed incorrectly, or to view incorrectly. Notice, I used the word "view" in both definitions. The same word, only given a second meaning. Likewise, you need only look to context clues to figure out which "version" of "mistaken" you are reading/hearing/speaking.

  • What about that. My readers always mistake me. That's why they write those angry comments. So: If I'm not mistaken (by readers) I have a higher ratings. Isn't it confusingly ambiguous? – Markus Marvell Dec 6 '16 at 8:21
  • @MarkusMarvell "My readers always mistake me" sounds weird. Better would be "My readers always misunderstand me". – Taemyr Dec 6 '16 at 12:48

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