What is the difference between slander and slanderous?

It depends on the context of the sentence you were writing. As your teacher said, slanderous is the adjective form of slander. So if you were using slander as an adjective, it should have been slanderous instead of slander. Slander by itself is a noun and sometimes a verb.

@Seaofclouds But what about the definition? He made sound like it changes too.

Slander is a noun, while slanderous is an adjective that describes things as being slander.

Like @PhiNotPi said, slanderous describes things as being slander, so it’s technically the same thing, but you still have to use the proper form in your writing.

Is CuriosityLoner the same as @CuriousLoner? No. An adjective describes a trait of its object, a noun IS an object. Syntax is important, your teacher was right giving you shit about it.

To say that’s “slanderous” language infers that what you are saying could be regarded as potentially damaging to the reputation of the entity that you are talking about. For something to be “slander” is that it is damaging to the entity you are talking about.

You talk untrue trash about a teacher to a friend, and it’s slanderous language. If you post what you said to Facebook, and it’s picked up by the newspaper, it’s slander.

@BarnacleBill If it is posted on Facebook, it would be libel as long as it is damaging and untrue.

I committed slander by saying slanderous things.

Slander is both the noun form and the verb form. You can “commit slander” (noun), you can “slander someone” (verb) and you can “utter a false statement that is slanderous” (adjective). And yes, it’s important. Verbs, nouns and adjectives simply aren’t interchangeable and the rules governing the use of nouns, verbs and adjectives is about as basic as the rules of grammar, syntax and usage get. You can communicate clearly or correctly without understanding the difference between the three parts of speech. And it gets harder from there.

I wanted add something but my internet connection failed so I couldn’t edit. You asked:

”...slanderous still means the same thing as slander correct?”

Yes, as mentioned, slanderous is the adjective form of slander just like in:

fame and famous
scandal and scandalous
virtue and virtuous
danger and dangerous