Chickens are fast not correct?
"Chickens are fast." is now accepted. 이/가 can also be the topic marker for emphasis though this usage is not introduced yet here.
Low level here. 닭은 would mean chickens in general. (이) specifies "the" chicken I believe.
Chickens are fast should be correct.
"닭이" sounds a bit like "딸기" here. I thought "닭" sounded like "dak". Why is there an ㄹ sound? Because of the 이? :o
Yes, it's because of the 이.
It's referring to a single chicken. If it was 닭는 then maybe you can say it's plural, because it would be referencing chickens in general
It would be 닭은, not 닭는.
Not putting "the" should be accepted