"Do all plural adjectives end with -en??" - Not necessarily. It depends on what type of article precedes the adjective and also on the case.
"is -en for acussative masculine only??" - No, it's used for other genders and for the plural as well.
The endings of attributive adjectives, i.e.adjectives before nouns such as "black cats", depend on three factors:
1) Type of article before the noun: der-words (=weak inflection), ein-words (=mixed inflection), unpreceded (=strong inflection)
2) Case of the noun (nominative, genitive, dative, accusative)
3) Gender/Number of the noun (masculine, feminine, neuter OR plural)
I.e. here, the noun "Katzen" is:
1) unpreceded, i.e. no article is used (=strong inflection)
2) Accusative (direct object)
Thus, the adjective "schwarz" gets the ending "-e".
What is this Select Word link?
Are you using the website version of Duolingo or a mobile app?
If it's a mobile app, is it iOS, Android, Windows Phone or something else?
Can you show me a screenshot and tell me where you try to click, what is supposed to happen, and what actually happens?
The sentence doesn't come up again today. But what I've been seeing is "Magst du ------ Katzen?" where I am supposed to choose between the words displayed in the drop-down menu. When I click on "Select Word," no choices appear. I am using the website version of Duolingo on my iMac computer. This same kind of freeze has been occurring on other sentences.
I'll hope it's that Duolingo had temporary problems that are now fixed!
They are still not fixed. If I delete the application and reinstall it, will I lose all my lingots? (I'm thinking that perhaps my application is corrupted.)