He is my brother.they are my sister 观察句子,为什么he后面用is而they后面用are?

( )is that boy he is my brother. He is my brother.对my brother提问 the baseball is ( )my brother The man is my brother.He is standing under the tree.用定语从句修改 My brother began to work when he was sixteen. My brother began to work ---- ---- ---- ---- sixteenHe is the oldest teacher in the school.He is ----- ---- ---- ---- teacher in the school. He is my brother.改为复数句 he is my brother 变成复数句子 He is my brother.改成一般疑问句 is,brother,he,my连词成句 brother my is he 连成句 (代词填空) is my brother.( he ) The man is my brother.He is taiking with Mary.合并成一个句子 The boy is my brother,________(he) name is Alan. 应该填什么 the computer game is that my brother is playing he favorite.我做的对么? the boy is my brother.He is wearing an orange T-shirt 【合为一句】 my little brother is a____ to sing in the class,He is shy The boy is my brother.(就my brother 提问) The boy is my brother 对my brother提问