"I am Roosa."
Translation:Minä olen Roosa.
I don't remember much of the Finnish I learned 50 years ago, but I do remember that Finnish divides vowels into 3 groups:
back vowels (a, o, u)
front vowels (ä, ö, y)
neutral vowels (e, i)
A word can only have front vowels or back vowels, so endings always have a front vowel or back vowel forms (ssä/ssa.)
I think a word with only neutral vowels (like mies*) would take endings with front vowels.
A is like the u in "cup".
The pronunciation of these letters is always the same regardless of the word.
Btw just a tip: if you don't have a keyboard with ä you can press ¨ and then a. For me it's to the left of Enter.
Here are instructions for it. The key is correct but you need to have US (or UK) international keyboard settings in use. I wonder how English talkers write naïve or did it become just naive after computers came more common. https://superuser.com/questions/1142924/umlaut-%C3%A4-%C3%B6-%C3%BC-on-english-us-layout