"Solid, liquid, and gas."

Translation: Στερεό,υγρό και αέριο.

[I.Schmidt1]

Solid, liquid, and gas. are these nouns or adjectives or nominalized adjective?
Στερεό,υγρό και αέριο. these are nouns!?
can I also use στερεά, υγρά και αέρια

[troll1995]

Both the English and the Greeks one are nominalized adjectives (so, they are nouns now). You can say στερεά, υγρά και αέρια but this translates as solids, liquids and gases.

[MoiraCalde1]

In Greek do you ever use the serial (Oxford) comma, so the answer would be "Στερεό,υγρό, και αέριο"?

[D_..]

We're taught not to use it before και (points are deducted if you do). In fact it's a whole category of conjunctions that cannot be paired with commas. While not explicitly mentioned in grammar books, all the examples like the sentence above do not have commas. Also teachers always make sure to drill this into our heads. :)

[MoiraCalde1]

Thanks for replying. I noticed that I lost a mark for the extra comma. In English both are correct, "A, B and C" or "A, B, and C".

[mizinamo]

I noticed that I lost a mark for the extra comma

That would surprise me; Duolingo completely ignores commas, as far as I know.

Do you happen to have a screenshot showing your answer being marked wrong for a misplaced comma?

I'm pretty sure that if you got marked wrong, it will be for something else than a missing comma. But people nearly never show screenshots so it's impossible to say what it was.

[MoiraCalde1]

No, I don't have a screen shot, so maybe it was something else that I failed on. Thanks for answering.