This is one of my go-to ones when it comes to how cases are applied: http://users.jyu.fi/~pamakine/kieli/suomi/sisallysen.html
https://uusikielemme.fi/ is one of my go-to ones when it comes to when you're supposed to apply whichever case.
Finnish cases often function as the equivalent of an English preposition, for example the inessive case ending "-ssa/-ssä" often corresponds to the "in" preposition of English. Consequently, Finnish prepositions are fewer and less commonly used than English prepositions.
And Duolingo probably has more about this on Tips, but the front vowels are ä, ö and y; and the back vowels are a, o and u. E and i are in the middle.
Front vowels cannot exist in the same word with back vowels (unless it's a compound word aka includes two or more different words; or a loanword which often makes them really difficult for the Finns to pronounce properly!) You can use that for telling if you need to use a or ä in the cases' suffixes because
- if you have back wovels only, then you use a: auto -> autoa (car).
- if you have front vowels, then you use ä: äiti -> äitiä (mother).
And as you can see from the latter word, e and i can exist with both vowel groups and those (back or front) will then determine if you need to use a or ä.
Words with only e's and i's are rarer but they also often use ä's in the suffixes.
Here's a little bit more about the vowel harmony.
On the website. Tips & Notes are on the desktop version of Duolingo. Every lesson you click on normally has two buttons: TIPS and START. First I always read the tips which is the whole introduction to this module, all vocabulary, grammar and examples. Then "start" has practice. That's why learning via phone only is not efficient.
Mämmi is not particularly sweet, many people load on sugar to make it so