Translation:I have another daughter, but she is older.
I thought the same thing. Just always think of it in terms of "What sounds better in English?" Although sometimes when I do this I get it wrong. It really is about exposing yourself to as much Italian as possible and letting it sink in. Remember, no one learns anything by not making mistakes. It is part of the process. Take guesses and see if you are right. If you are not then try again. CIAO!
I have just seen a similar discussion in the Dutch course because Dutch and German use "bigger" with children in much the same way as Italian does. Which happens to be the opposite of how English uses "little" with children. A "little" child is small, young and immature all at the same time, with none of these aspects really dominating. Similarly, for a child to be "più grande" means to be bigger, older and more mature all at the same time. The best one-word approximation in English is to say "older" because it sort of implies the other two.
I translated it as bigger and it was accepted. Grande as older makes some sense. It would be similar to a girl saying "This is my big sister".
Is there a rule for when you use an apostrophe (un'altra)? I feel like sometimes words are contacted and then sometimes not.
Just like in English, the apostrophe indicates that a word is shortened by one or more letters. In this case, una altra was contracted to un'altra, removing the a of una. I don't know if this particular contraction is obligatory, but it is certainly very common because una altra is a bit inconvenient to pronounce with its clash of two a's.
That's how you can talk about your elder daughter if she is over 60 or so. It implies that she is actually old, not just older than a sibling.
No. You use "elder" as a noun not an adjective. "I have two daughters the elder is twelve" In fact "I have two daughters the elder is older" is perfect grammatically.
A minor correction: The key information is that (1) elder is a variant of older that can only be used with a person, and (2) it is never used predicatively. Example:
- I have a younger sister and an elder sister. I call the elder elder because she is older than me.
In the example, each instance of elder could be replaced by older. But the one instance of older can't be replaced by elder because it's predicative. Note that in "elder sister", elder definitely functions as an adjective (used attributively).