"A big dog is bigger than a small one."
Translation:Ein großer Hund ist größer als ein kleiner.
I don't remember seeing anything about inflections in any of the tips? I had to look that up about strong, mixed and weak. So, now we've got to look at what the subject is, what the direct object is , what the indirect object is possibly, then what gender the nouns are, if nominative, accusative, or dative applies (or genitive, haven't' even gotten that far yet) plus apply specific endings to adjectives and then also through into the mix inflection endings all in a split second when developing a sentence. Would be easier for me to literally memorize every possible combination than try to figure that out. Nothing again duolingo, just languages in general.
You are right, sadly doulingo does not talk about strong, weak and mixed inflections. You can find all about them here: https://en.wikipedia.org/wiki/German_declension#Attributive_adjectives
It takes a while, but all these exercises make more sense after learning them
Why is the "als ein kleiner" not in the akkusative?
Why would it be?
You're comparing it to ein großer Hund, which is the subject of the verb ist and is thus in the nominative case.
So ein kleiner is also in the nominative case.
The thing after als is in the same case as the thing you are comparing it with.
Compare, for example, these two translations of "I love you more than my father":
- Ich liebe dich mehr als meinen Vater. (meinen Vater is accusative, so you must be comparing it with accusative dich = I love you more than I love my father)
- Ich liebe dich mehr als mein Vater. (mein Vater is nominative, so you must be comparing it with nominative ich = I love you more than my father loves you)