Noun x Adjective

[Emanuelli]

First, I would like to know if there is a limit of questions I may ask. I mean, I've already used this site to help me many times and I wonder if I can do this without limiting.

Second, in the sentences:
a) They are American. / We are catholic.
b) They are Americans. / We are catholics.
When should I use the plural form? Can I say that in the first examples, American and catholic were adjectives and that in the second one they were nouns?

Which one is better? I've already seen both of them.

Thanks.
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[Casiopea]

First, I would like to know if there is a limit of questions I may ask. I mean, I've already used this site to help me many times and I wonder if I can do this without limiting.

You can ask as many questions as you like, but try not to put them all into one post.

Second, in the sentences:

a) They are American. / We are catholic.
b) They are Americans. / We are catholics.

When should I use the plural form? Can I say that in the first examples, American and catholic were adjectives and that in the second one they were nouns?

Which one is better? I've already seen both of them.

The forms in a) are adjectives. They're short for, say, "American (people)", "Catholic (believers)". Use the plural forms in b), the nouns, to rename the subject, and the singular forms in a) to modify the subject. Adjectives tell us what kind of people they are:

They are happy (people)
They are American (people).
They are Americans. ~ They are American people.