When do I use doi instead of doua?
when the noun is masculine. doi is masculine and doua is feminine.
How do we type the accented characters on a macintosh keyboard?
You can go to System preferences > Keyboard > Input Sources > Add Romanian Keyboard.
Duolingo also provides those special letters under the input box.
Groovy! I could make a song out of this...
There's no way to type the accented characters on a phone. Is there?
yes there is, set your phone's input language to romanian and hold the letter you want with an accent, for example 'a', and a menu will open to select either ă or â.