This is such a common complaint that I’m only going to offer a single example of it, and leave it up to you whether you want to waste part of your life looking up other examples. From the BBC’s idiotic list of “Americanisms”:

16. “I’m good” for “I’m well”. That’ll do for a start.

There is nothing wrong with “I’m good”. And yet, this is the sort of grammatical myth that not merely persists, but pervades. One of my best friends in college ragged on me for it. One of my current friends (an English teacher no less) subtly corrects me for it regularly.

There are a few reasons why people might think that I’m good is incorrect. The most prominent, the one I’m often given as justification, is that good is an adjective and well an adverb. That’s all well and good, but am is a conjugated form of to be. To be is a linking verb here, which means that it takes a predicative adjective, not an adverb. We say things like I’m hungry, not I’m hungrily. An adjective is what you need here, without question.

Of course, well isn’t only an adverb; it can be an adjective as well. That leads to the next argument against I’m good: that good is an adjective, but it’s the wrong adjective. For instance:

When you ask an American: “How are you today?”, they say: “I’m good” (Meaning: I’m a good person) when they should use “I’m well” (Meaning: I’m fine or healthy or something like that).

But to get the “I’m a good person” meaning out of I’m good, you have to try to misinterpret it. Sure, saying I’m good can be interpreted as “I’m not evil”, but that’s far from the only possible meaning, and it’s hardly the most reasonable. I don’t want to be condescending, but even a non-native speaker of English is aware that good has a lot of possible meanings. Here are two from the Oxford English Dictionary:

1. Of persons, as a term of indefinite commendation.

2. Such as should be desired or approved, right, satisfactory; sound, unimpaired; not depressed or dejected.

Those senses of good, which date to 1154 and 1175, respectively*, are more likely intentions when responding to “How are you?” than an unsolicited assertion that one is a moral human being. To say that the “moral” meaning is either the only acceptable one or the most reasonable one in this context is to say that you do not have a good grasp of the English language.

So I think that that establishes why I’m good is acceptable, and really does mean “I’m fine”. But perhaps I’m well is more acceptable? Hey, maybe for you it is, and if it is, godspeed. But for me, the two forms have significantly different meanings, and in general I mean to say that I am good when I say I’m good.

I’m well means that I am healthy, which I almost always am if I’m wandering around talking to people. When people ask, “How are you?”, they’re not, in general, inquiring about your state of health but rather your state of mind. Thus I respond that I am good, in that second definition above, feeling right, satisfactory, unimpaired, and neither depressed nor dejected. I do not respond that I am well, because I think that’s pretty obvious, and if it’s not obvious I’m well, it’s likely because I am unwell.

I think that most people feel the same; when my friends tell me that they are good, they tend to follow up with something like “I got a new video game” or “I’ve been enjoying this weather”, indications not of good health but of good feelings. That’s not to say that you shouldn’t say that you’re well; you are welcome to. This is only why I don’t say I’m well.

I want to talk about two similar situations — I’m feeling good and I’m doing good — as well as whether I’m good is too vague, but I’ve gone on long enough. I’ve put together a second post discussing I’m feeling good and I’m feeling bad, and hope to finish off with one on I’m doing good in the future.

Summary: I’m good is correct, because am is a linking verb, taking an adjectival predicate, and good is that adjective. I’m good means that one is fine, in good spirits, etc. I’m well is fine too, but I find it to focus more on one’s health than general state of being.

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*: And, of course, they’re attested through the modern day.