De acordo com os estudos do verbo “to be”, todas as frases estão escritas gramaticalmente corretas, EXCETO:

• Are Paul and Mary your parents? No, they aren’t.

• I’m not an American boy. I’m from England.

• Is Bob and I friends? Yes, he is.

• Is the lion at the zoo? No, it isn’t.

• Is Claire a good student? Yes, she is.