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?
A1
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".
A1-1
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".
A2
"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.
A3
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.
