Elephants have a long nose(trunk) (correct)
An elephant have a long nose(trunk) (An elephant has a long nose.)
Elepahnts have long noses(trunks) (correct)
Elephant has a long nose(trunk) (An elephant has a long nose.)
Birds have a beak. (correct)
A bird have a beak. (A bird has a beak.)
Birds have beaks. (correct)
Bird have a beak. (Birds have a beak.)
I hope this helps! :)