This is a good question! Part I: The difference between 'she' and 'her' is possession. 'She' is the same as 'ella' in Spanish, while 'her' is more like 'su' - but has a feminine gender. Part II: This one is harder. Here, the first sentence has an independent clause and a dependent clause. I will break the first sentence up into the two clauses, but it may be a little confusing. Tell me if you would prefer me to rewrite it. It was them (An independent clause - in this case the verb object is 'them'). who called this morning. (A dependent clause - this clause, while a complete sentence, does not express a complete thought in this example.) 'Them' is used as an object of a verb - in this case, the object of the verb 'to be'. The second sentence is much easier, because it is not a complex sentence. 'They' is the subject, and it is used because of a rule in English called 'subject-verb agreement'. This means that the subject's tense and number must agree with the verb's tense and number. 'Were' is the plural form of 'was', and 'they' is a plural noun that indicates that the speaker is not a part of that group. I hope that helps. English grammar is confusing (especially subject-verb agreement) even for native speakers. I had a very hard time with it in college.

Part I - It was she who called him first. - It was her who called him first. Native speakers use both these constructions. The first is the grammatically correct form. 'She' is the subject of the sentence, and 'she' is the correct subject pronoun. However, in casual conversation, it is common to use the second and less grammatically correct form. It sounds more natural and less formal. Part II - It was them who called this morning This is the same as the second sentence above. It's an acceptable form in informal speech. The more grammatically correct form is 'It was they who called this morning.' - They were who called this morning This makes no sense, I'm afraid. The sentence is not complete, because the relative pronoun 'who' has nothing to relate to. If you want to begin with 'They were', you would have to use a complete relative clause. 'They were the people who called this morning.' or 'They were the ones who called this morning.'