"My little brother thinks penguins are mammals!"
Translation:Mein kleiner Bruder glaubt, Pinguine sind Säugetiere!
Should be possible with and without, but since there is no "that" here, I wouldn't use "dass"
- "....., Pinguine sind Säugetiere".
- "....., dass Pinguine Säugetiere sind".
Because adjectives in attributive position (i.e. modifying a noun, coming directly before it) are declined according to case and gender in German (while they are not in predicative position, like in ‘mein Bruder ist klein’). In this case, ‘Bruder’ is masculine and it's the subject of the sentence, which means it's in the nominative case. Since the noun is modified by an ein-like determiner, the nominative masculine singular takes the ‘strong’ ending: -er.
It's a lot to take in all at once if you have no knowledge of German adjectival declension, but you can break it down like this:
Does the noun have a determiner or article? Yes (in this case ‘kein’).
Does this article have an ending? No → the adjective must carry the ending with information about gender, case, and number, that is the strong ending (which corresponds to the endings of the definite article ‘der, die, das’).