When do you use "I don't know either" and when "I also don't know" (or any other verb instead of "know")?

I've been taught that "I also don't know" is not as correct as "I don't know either" - just like "I always don't know" is not as correct as "I never know" - but I've been hearing recently native speakers use "I also don't know" quite often.

Perhaps "I also don't know" is also correct, but it has some different meaning?

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    As a native AmE speaker, I have no problem saying "I also don't know" if the person asked "What is ...?" right before me says "I don't know" (if in fact I don't know). It sounds just as correct as "I don't know either". – Mitch Sep 11 '17 at 22:24
  • A asked B something, and B replied, "I don't know.", Then asked C, C shoudl reply "I don't eigher or I also don't know?" – Zhang Jan 19 at 6:49

I can't think of any circumstances in which "I also don't know" would be correct as a standalone phrase. I'd always use "I don't know either".

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    Right, I suppose you only use it when there are several things you don't know: "I don't know what the animal is doing in here. I also don't know how it got through the fence. We need to know this before we can act." In a more formal or traditional setting, you could say "nor do I know how it got here". – Cerberus_Reinstate_Monica Dec 24 '10 at 1:45

"I also don't know" implies that not knowing is one of a list of attributes that describe you.

Instead of "I don't know either" you could say "I too don't know." This puts the emphasis squarely on the subject instead of on the action.

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    Is "I too don't know" correct? I've never heard people say that. – brilliant Dec 23 '10 at 20:51
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    @brilliant: It's a little bit formal, more so than "I don't know either," but it's certainly correct grammar. Consider a parallel construction like "I too work in finance." Putting the "to"after the subject makes its meaning clear. "I work in finance too" could mean working in finance is merely one of the many things you do. The meaning may be clear from context, but if it is not my suggestion provides a way too make certain it is. – Robusto Dec 23 '10 at 21:14
  • @brilliant: I use that form from time to time. More often in writing that speaking, however, because it is pretty formal. – dmckee --- ex-moderator kitten Dec 23 '10 at 22:10
  • If we can say "I too don't know", can we say "I don't know, too" or "I didn't know, too"? cc @dmckee – baptx Sep 11 '17 at 20:22
  • @Robusto How is "I too don't know" different from "I also don't know". They mean the same thing to me, but 'too' sounds a little stilted there to me. – Mitch Sep 11 '17 at 22:26

If someone has stated that they don't know something, and you are stating you don't know that same something, you would would use "I don't know either."

The only situation where "I also don't know" would be correct wouldn't be standalone. If you had previously stated that you do not know, you could say "I also don't know _." This would be correct.

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