why is "A faca é bom" not correct?
Good = bom/boa. Bom is used for maculine words and boa for feminine ones. Their plural forms are bons and boas
In another question, it was "Manteiga e bom" and, even though "manteiga" is feminine, it was correct to use "bom" in that sentence, but here, we have to use "boa".
it is related to the artcicle. When you have the artcicle, bom must agree in gender and number. Manteiga é bom. A manteiga é boa. A faca é boa.
faca is a "she" so the adjective should fit the word FACA
what if you say o facão é bom (which is correct and means the same thing)
"Facão" is just for big knives, like a dagger or something. Then, it would be a masculine word. If you want to refer to a kitchen knife, it is: "a faca".
what about "bom dia".... isn't "dia" feminine? Thankfully choose "boa" and kept my heart!
Yes, boa is used for feminine words. Although dia ends wih an A, it's a masculine word, and the you use bom,
What about bem?
Here the word bom(adjective) is related to the faca(noun). Bem(adverb) you use when it is related to the verb! =)