Christian-- absolutely agree with "Lehrerin", but the program insisted that "sie ist Lehrer" is correct. For me this was a broader, new usage of the word "Lehrer" and like you, I learned "Lehrerin" as well.
One last question please. Consider the sentence, "Frau Scmidt ist mein Freund;sie ist auch meine Schwester." Would you possibly write, "es ist auch meine Schwester"?
>For me this was a broader, new usage of the word "Lehrer"
Right. I only meant to remove the indefinite article. Personally, I'd use "Lehrerin" though. Nowadays, using the masculine form of a job title to refer to a woman is generally discouraged if a feminine form is available. http://en.wikipedia.org/wiki/Gender-neutrality_in_languages_with_grammatical_gender#German
>Would you possibly write, "es ist auch meine Schwester"?
No, I wouldn't. Correction: Frau Schmidt ist meine Freundin. You definitely can't use the masculine form here.
"When it is used as a predicative, es can refer to singular and plural nouns of all three genders and to adjectives. The verb agrees with the subject, not with es"
"Es" is pretty much required. I'm struggling to find examples where "sie" is okay. In most cases, "sie ist eine Zeitung" sounds just as odd as "she is a newspaper". Here are two examples where "sie" is used.
"Diese Textsammlung ist kein Buch; sie ist eine Zeitung." ("sie" refers to "Textsammlung")
Some surreal play where a female portrays an actual newspaper.
Right. "sie" is correct in this case. "es" would not work. The first example I gave works the same way ("Textsammlung" is a feminine noun).
To complicate things even further, I'd most likely use "die": "Hast du diese Zeitung gesehen? Die ist aber groß!" You're probably not aware of this, but "der", "die", and "das" can be used as demonstrative pronouns. To me, "die" sounds slightly more natural than "sie".
http://canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-der-die-das.html?lang=en (see "Anaphoric reference")
I'm completely convinced that "es" is a-okay, and that "sie" is not. But I do have a related question on this. Is "sie" okay to refer back to an inanimate object that's already been introduced, but happens to be a female noun? I've always thought that it was, but I may be wrong, and I know you know.
As an example, say I wanted to say something like "Have you seen this newspaper? It's big!" Not really knowing what I'm doing, I might try something like "Hast du diese Zeitung gesehen? Sie ist aber groß!" where I use "sie" in the second sentence to mean "it". Or does that have to be "es"? Or would "es" even work there?
I note as well that "es" used this way in "Es ist eine Zeitung" is with a form of "sein" or a transitive verb form--third person singular--. My example just above is with an intransitive verb--other than all the rules shown in your link, could that be the simplest answer to my confusion?