Are they teachers ? They are not teachers. I am not a thief. They are thieves. 'Are' is a plural form of the verb 'to be' therefore the noun must agree with the verb. (i.e. the noun must become plural) When using the singular form of 'to be' you must put a determiner before the noun.

They = plural pronoun He = singular pronoun Ex: They are teachers (plural). He is a teacher (singular).

they are teachers.they are not teachers.i am not a chief,they are thieves