It's spelt differently than the singular, it sounds different, and it's not preceded by any article.
an apple = ein Apfel /ˈapfl̩/
apples = Äpfel /ˈɛpfl̩/
In addition to Treeman22's answer. If you have a Mac you can use Alt-U then type the letter you need (a, o, u, A, O, U). If you use Alt-S you get ß. Windows has similar keystrokes but I'm not sure they are the same.
On iPhone or iPad you can simply hold down the relevant key and after a second you will get a full range of accented versions (e.g. hold A and you'll be able to select Ä from a popup). This also works for currency symbols and other items on the keyboard like .com will pop up a list with .net, .co.uk, etc..
Hope these tips are useful to anyone reading :)
If typing German letters is inconvenient for you, you can type "ae" instead of "ä", "oe" instead of "ö", "ue" instead of "ü", and "ss" instead of "ß". This is what native speakers do when they're abroad. If you do this, Duolingo might give you a message showing the proper spelling, but you shouldn't lose a heart.
Take a look through your Windows keyboard settings. There's a US International keyboard that includes a whole bunch of nifty shortcuts. E.g., double-quote then a vowel--capitalized or not--produces the vowel with an umlaut.
Edit to say disregard most of this sentence; the problem was in one of my add-ons: Right-alt S is supposed to produce the double S/beta/ess-tset character, but it's not working for me in Firefox at the moment, which is how I happened to stumble across this conversation.
Won't let me reply to your next post, thanks, that helped me find what i needed, i think i was just too tired the other night.
I'm not sure i want to memorize a new keyboard layout right now though! It probably would be worth the effort, but i'm going to download Logitech's SetPoint (since i have a Logitech keyboard) and see if that works for me. The German keyboard uses the ";" "'", "[", and "-" keys for the umlauts and ess-tset for example, and i would much rather reassign "[", "]", and back-slash to those umlauts. I'm not sure where i'll put the ess-tset yet. But i use the ";" and "''" too much to remap that comfortably. The German keyboard also switches z and y! Might be fun to figure that out in the near future.
Thanks for your help.
ETA: SetPoint didn't let me program as many keys as it promised, and none to umlauts, i'm just using the German keyboard now.
Meine is used for plurals and feminine. It does refer to my daughter in this instance, 'meine Tochter' because 'Tochter' is a feminine word. If it was referring to a masculine word here, 'mein Bruder' it would be 'mein' instead. 'Mein' would be used for masculine and neuter words while 'meine' is reserved for feminine and plural words.
It depends on the context. If you want to call your daughter your "girl" you would say "Mädchen". If you want to make it clear that the girl is your daughter, you would say "Tochter". Neither one is better than the other, but the context should be clear that you are talking about your daughter instead of your girlfriend.