If a person is a student, then he or she must pay attention to the strictly formal grammar. Strictly speaking, "this is she" and "it is they" is correct grammar. However, people often use the objective case pronouns, her and them, especially when speaking.

I can't formally explain the grammar, but you will probably never be wrong if you use 'This is her' and 'This is them', but you would almost never use 'This is she' or even more so 'This is they.' In each of these instances you would probably clarify the subject and object by saying 'She is the one' or 'These are the ones...' 'This is she' and 'These are they' may be technically correct, but you should simply avoid there use. In my 50+ years of speaking English as a native, I've probably used those phrases once or twice and only in some formal, literary contest. Hope that helps.... Erik

maybe for example: this is her ball. but the ball is hers.