Good is an adjective. Well is usually an adverb, but it can act as an adjective sometimes.
These two words are involved in many of the corrections amateur grammarians attempt to make. When someone asks me how I’m doing, I usually reply, “I’m good. How are you?” I can’t tell you how many times the person has come back with, “I’m WELL, thanks,” hitting that well like they’re spiking a volleyball back in my face. (Also see Can vs. May.)
To see why “I’m good” and “I’m well” are both perfectly acceptable responses, we’ll take it one word at a time.
Good is an adjective. Adjectives modify nouns. You can have a good day or a good time. You can eat a good burger or drink good coffee. Day, time, burger and coffee are nouns, so there’s no debating that we should use adjectives to modify them.
When people say, “I am good,” they’re using good to modify I. Because I is a noun, this use of good is perfectly correct. The confusion occurs because the presence of the verb am makes people think we need an adverb. For example, you might say, “I play the piano poorly.” The adverb poorly is modifying the verb play, so that sentence is correct. But in “I am good,” good is modifying I, not am. Therefore, good is correct.
Well is an adverb. Adverbs modify verbs, adjectives or other adverbs. To use the piano example again, you might say, “I play the piano well.” The adverb well is modifying the verb play. You could also sing well, dance well, speak well and do many other things well.
However, well can also be an adjective, usually to describe someone who is in good health. When someone is sick, we say, “Get well soon.” So, when people say, “I’m well,” they’re using well as an adjective modifying I, even if they don’t realize that’s what they’re doing. They’re essentially saying, “I’m in good health.”
To sum it up, if someone asks you how you’re doing: “I’m good” is correct. “I’m well” is correct. Carry on.
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