The answer is quite simple if you look at the number of words. �When it comes to language the British but particularly North Americans are lazy. �Therefore

2000�Two thousand (2 words)�Twenty hundred / twenty zero zero / twenty oh oh (2 or 3 words)�Here we make the distinction because 2000 marked an important date

2008�Two thousand and eight (4 words)�Twenty oh eight (3 words)�

There isn�t a noticeable difference until we get a few more years back or forward:

1998�Nineteen ninety eight (3 words)�One thousand nine hundred and ninety eight (7 words)

2010�Twenty ten (2 words)�two thousand and ten (4 words)

2021�Twenty twenty one (3 words)�two thousand and twenty one (5 words)

My personal prediction is that people in general will use twenty ten in 2010. �However if it doesn�t occur then it will definitely occur in twenty twenty 2020, because it�s already a used expression (20/20 vision) and it has a nice ring to it. �This is what I tell my students, but really the only answer is to wait and see, even though one form or other may be incorrect, it really depends what joe blow will say, therefore count the number of words because joe is lazy.

For extra lazieness remember that North American don�t use "and" in their numbers (two thousand eight)

I�m not basing this on anything official but only my own feeling as being English and having lived in London all my life til this summer....

I have 3 general thoughts:

1. All languages are made up of units of meaning. Spoken languages are also made up of units of sound. In every language & dialect there are sound formation patterns that are followed - eg an apple, an MBA, a dog, a degree - the common pattern here is that �an� always precedes a vowel sound (not letter) and in all other cases �a� is used.