What's the rule for using a/an before an adjective?
I am asking this question because my high school professor is teaching us that we shouldn't use a/an before an adjective. With some 'exceptions' like "Picasso was a famous painter."
Now that sounds very strange to me because I am used to saying "It's a beautiful day.", not "It's beautiful day.", or "It's a small dog.", not "It's small dog". My professor also pointed out that "Picasso was a famous painter." was some kind of exception because "a" is used before an adjective. My professor also told me that this sentence is correct : "A mouse is an animal. It's small animal."
Like I said, for me, It's a very strange claim that we shouldn't use "a" or "an" before an adjective. What should I say to my professor? I would like your professional opinion about this.
Notice: This question isn't about 'a' vs 'an'. Read the question before marking it as a duplicate.