Translation:There is a dog under the table and a cat under the chair.
"A dog is under the table and a cat under the chair." Why do you consider this wrong?
You started with "a dog is", so there is "is" for the cat part missing. The sentence after "and" does not contain a verb. "There is a dog […] and a cat […]." works, as it is no more two sentences, but a list (a dog, a cat,… )
And why is it bad English? Not all who learn Esperanto are native speakers :) (Dutch)
Because in “good English” you say: “There is a dog …/There are a dog and a cat …”
That is not a question of being a native speaker or not, but of the education level.
Does it mean that in Esperanto one may use the verb esti without a person of the verb and then it means: there is/ are? What about other verbs: exp: kreskas plantoj kaj bestoj. How would I translate that into English?