“Future” can be either an adjective or a noun. When used as an adjective, it doesn’t take any article itself; it is preceded by the article of the noun it modifies:
The card will be sent to you at a future date.
This policy will affect the future course of action.
We do it for future generations!
Of course, the logic stays the same even after the preposition “in”, which is probably the most confusing case for non-native speakers:
I would like to address this issue in future articles.
When “future” is used as a noun, the situation gets a little more complicated. When “future” means “the time or the events that will come after the present”, it is always used with the definite article:
correct No one knows the future.
wrong No one knows future.
correct You should start thinking about the future.
wrong You should start thinking about future.
The phrase “in the future” in AmE and BrE
The phrase “in (the) future” has two meanings. When it means “at a future point in time”, it is used with the definite article:
correct I would like to move to Spain in the future.
wrong I would like to move to Spain in future.
However, when it means “from now on”, there is a divide between American English and British English. An American would still say “in the future”, as in the previous case, whereas a Brit would likely say “in future” (with no article). Thus, “from now on, please, be more careful” could be rephrased as
In future, please, be more careful. (British English)
In the future, please, be more careful. (American English)
If you speak American English, you don’t have to care about the distinction. However, if you speak British English, using “in future” instead of “in the future” can completely change the meaning of a sentence. Compare
Human beings will live on the Moon in the future.
(Human beings will live on the Moon at some point in the future.)
and
Human beings will live on the Moon in future. (British English only)
(Human beings will live on the Moon from now on.)