[MTB ]How much can my bike endure? I recently bought a (Mountainbike) Bergamont Revox 3.0 and I'm concerned that I'm not taking proper care of it.

I want to know if I can, for example, ride down a flight of stairs or switch the gears while I'm riding up a steep hill.

I clean it regularly where I can, and it's currently sitting in the basement (due to weather) anyway. 

But the question is, how much "rider-abuse" can my bike withstand ?
Bikes break because they've been either mistreated or not cared for.  Riding a bike down stairs should be okay provided you don't fall off or catch anything.... that's what a MTB is built for.

Proper care and maintenance keeps everything working, so lubrication and washing off mud/silt etc will keep the bike functioning.  Storing your bike inside out of the weather is the single best thing you can do to preserve it and extend its life.  

If your question was about a road bike or a hybrid/commuter, I'd say stairs would be outside the normal usage for such a frame.

Finally, every bike needs to change gear while going uphill.  The only trick is timing, to get the chain to move cogs/chainrings while at the top of the stroke where the pressure is less.  If you're changing gear with the pedal cranks horizontal then that's sub-optimal and bad for the transmission.

This question is likely to generate various opinions.