Well, strange sentences are great for memorizing. You'll never use these sentences anyway, but by them you are learning the language and you'll recreate some of it later.
It is like mathematics, you learn how to compute angles, but 99% of us will never compute angels - but we are learning how to think.
When Ceasar led his army over the River Rubicon, said "iacta alea est" or "Alea iacta est". It wasn't a remarkable piece of poetry but stayed up to date because it was good to memorize a certain case in Latin.
Strange sentences helps you to focus on the grammatical rule instead of a common small story in your head, with what you you'll forget the sentence anyway, because it is routine and you are focusing on the imagined situation rather on the grammar.
You asked about the "-in" ending. Freundin is a singular noun for a female friend. Lernerin is a singular female teacher. I can't recall if they have plurals, e.g. Freundinen.
So, "die Freundin" is accusative feminine singular in this usage.
--- that's the gist, now far too much more :) -----
I'm not sure what your native language is, but I only can give examples in English: e.g. waiter/waitress, steward/stewardess, and obsolete forms like doctor/doctress (yes, really). And a general tendency to use single person male "he" as the "neutral" personal pronoun. It's a grammatic area that has evolved in English with more gender-balance awareness, and we see fewer and fewer, with a tendency to collapse onto the male noun (waiter) or a novel word (waitron) or some other expression, such as flight attendant for steward/ess.
I'm not sure if the same has been true in German. I wonder if the presence of gender markers on all nouns makes this less of a social issue in German? Anybody want to comment?