we say the sixties, the seventies etc. how can we say 2010 in this way? and is "zero decade" correct? and 2021 for example, is it twenty-twenty one, or year two thousand twenty one? thank you.
2021 is a matter of personal choice. Some people will say twenty twenty one, others will say two thousand and twenty one, or two thousand twenty one.
Your first question is more tricky. Some people refer to the present decade as the "noughties", following on from the nineties.
People say "the nineteen hundreds" to refer to the years 1900-1910, and "the twenties" for the 1920-1920 decade, but I've never heard a term for the years 1910-1920.