2011, the start (the first year) of a new decade
Let's suppose we are going to count a large quantity of things.
It doesn't matter what the "things" are.
They may be fossilised cow-pats, 1.74 litre tins of Polka Dot paint or little pieces of fluff which you have excavated from your navel.
But for our purposes it's probably better that you think of years, so that you don't get confused (as most people obviously are).
And we are going to count the things in units of ten (10). That's what decades are; sets, series or groups of ten. Notice the ‘dec’ of decade, and the ‘dec’ of decimal – things based on 10.
The first item of the first decade will be number 1.
The last (the tenth) item of the first decade will be number 10.
Is that clear? If not, read it again and continue doing so until it is.
The first item of the second decade will be number 11.
The last, and tenth, item of the second decade will be number 20.
The corresponding numbers for the following decade will be 21 and 30, etc, no exceptions.
If you carefully inspect those numbers you should be able to see that the last digit of the first item of all decades is always 1 (one).
You should also be able to see that the last digit of the last item of all decades is always 0 (zero).
The same is also true for centuries and for millenia (more than one millenium).
Last year, 2009, was the ninth year of the present decade, NOT the tenth.
This year, 2010, is the last year of a decade.
Is that REALLY so hard to understand?