Ein super Restaurant

Where did that come from?

It has nothing to do with what letter the adjective ends. "Super" is an adjective of foreign origin and it is non-declinational. Other examples are "extra" or "prima". As often there are exceptions as well and "cool" is such an adjective. There are also German adjectives that are non-declinational, e.g. "weniger".

I had second thoughts about those foreign adjectives. Could it be that there is a distinction depending on syllables? Maybe short worts with only one syllable do change while words with two or more syllables do not? This would explain the difference between super and cool. Just guessing here and trying to think of other examples but nothing comes to my mind right now.

Cosmopolitan, no, it does not have something to do with syllables. And "weniger" is declinable:
wenig, weniger, am wenigsten; ein paar wenige Euros; nur wenig Geld....

super is indeed special, it is just used in this form. German is quite complicated, we do have a lot of exeptions. It is just like the irregular verbs in English, you need to remember them...

I realize that syllables do not trigger declination. After some more thinking "top" came to my mind and it only has one syllable but is non-declinational.

I am not too sure about "weniger" because I was not referring to the positive, comparative and superlative. The comparative "weniger" itself is non-declinational - at least I cannot think of an exception.

No, of course the comparative weniger is not declinable, but I gave you two examples which show thet the word "wenig" is...

cocolino18: "of course the comparative weniger is not declinable". OK, but why? Comparitives usually decline. So what's this "of course"?

Cosmopolitan #11, "extra" and "prima" end in vowels. So naturally they don't decline -- they can't. It looks like you didn't read my OP either!
But "top" is a good example of the "super" phenomenon.

Two observations:
> - Top as in " ein top Hotel" certainly exists. BUT the declined version "ein toppes Hotel", in which the world feels much more adjectivy to me, is not as impossible as "eine superes Hotel".

• While super ends in a consonant in the written form, the sound it ends with is actually a vowel, not unlike the a in prima and extra.

I know there are German adjectives ending in the same sound which decline perfectly well (ein leckeres Bier), but they are not foreign. Could this be some kind of explanation? Foreign adjectives that end in a vowel SOUND do not decline like native adjectives.

Mmh, I'm sure someone will come up with an example that will prove this wrong as well.

Oh, and by the way, people do actually try to decline adjectives like lila - "eine lila(n)e Hose", just like "ein rosanes Haus", the latter being quite acceptable. Does not work for prima and super, though.