"Die Hose" can be used in the singular, unlike in English. So "Der Mann trägt eine Hose" meaning "The man wears pants." If you use the plural ("Hosen") it literally means multiple pairs of pants. But I have also heard that it's possible to use the plural when you just mean one pair pf pants, like in English. I'm not sure if there are any situations where one would be more correct than the other though. It seems slightly more rare to use the plural, but I don't know for sure. There's a pretty interesting discussion about it here: http://german.stackexchange.com/questions/3980/when-do-we-use-hose-like-a-plurale-tantum
- Klein isn't declined because it is a predicate adjective.
- The umlauts change the pronunciation of the vowel--and thus the word--so that in the spoken language one can tell that there is a change in conjugation. It's sort of a "that's just the way it is" situation. (Es ist halt so.)