Re: what is the difference between " in twenty years" and " in the twenty years"
Actually it is fairly straightforward...
"In 20 years" does not have a Relative/Adjective Clause after it. It is a complete Adverb Clause (which may or may not need a comma after it--but it often does).
"In the 20 years" always has a Relative Clause after it! (NOTE: this phrase is almost always talking about a time in the past)
Both can be used to talk about the past and future.
(1) In 20 years, I have not seen a black bear in my neighborhood.
(2) In 20 years, everyone will have a flying car.
(3) In the 20 years since the creation of the internet, more people have learned wanted to learn foreign languages than ever before.
(4) In the 20 years (that) it took me to write my first Sci-Fi novel, many things I write about in the novel have come true.
Sorry if this comes too late to help you trankhaihoan...I just joined the site!
Chris