Anki is a tool that helps solve one of the great challenges when trying to learn something over the long term: forgetting. One common reason we forget what we have learned is that we don't review it frequently enough. For example, in primary school you might have spent a lot of time learning how to do different forms of mathematical operations, but if you haven't actually used these operations since then, it is quite unlikely that you remember how to use them. However, reviewing takes time and energy, so we don't want to spend too much time on it either.
The way Anki solves this problem is by keeping track of when is the optimal time to review past knowledge. The program does this by estimating when there is a ninety percent chance that you will be able to recall a piece of information correctly, and at that point it prompts you to try to do so. Of course, it is not certain that you will be able to recall it—after all, there is a ten percent risk that you will not—but every time you try to do so, you reinforce and strengthen your memory of the piece of knowledge, which means you will have a greater chance of recalling it in the future.
The principal driving all of this is called "spaced repetition", which says that when we wait a bit before we try to recall a piece of information, the amount of time it takes for us to forget it increases dramatically.
When starting out making flash cards, many tend to put a lot of information into each card. For example:
What are birds?
As you may see, this can get overwhelming quite quickly. Studying these kinds of cards is both tedious and frustrating. The alternative is to opt for making multiple, very specific cards, which would look something like this:
What are birds?
A group.
What are birds a group of?
Dinosaurs.
What clade of dinosaurs do birds belong to?
Theropods.
What class do birds constitute?
Aves.
What kind of blood do birds have?
Warm blood.
Which visual features charectarises birds?
Feathers and toothless beaked jaws.
How do birds reproduce?
By laying eggs.
How are bird eggs?
Hard-shelled.
How are birds' metabolic rates?
High.
How many chambers do birds' hearts have?
Four.
How are birds' skeletons?
Strong yet lightweight.
This way, each piece of knowledge is isolated into its own card, which means that, when we are reviewing, we won't get overwhelmed. Before, if there were a single a piece of the answer that we did not know, we would have to say that we failed to recall the entire card correctly. Now, we only have a to review the pieces of knowledge that we don't know, instead of all the pieces of knowledge that were in the original card.
Does this mean that we can only make flash cards with one word answers? No, the point is that we don't want to overwhelm ourselves with too much new information at the same time, when there is no need to. For example, let's suppose that we're trying to learn the english sentence: "There are three beautiful candles on the table in the kitchen." If we already know all the words making up the sentence, but want to learn that "on the table" goes before "in the kitchen", you could make a card like this:
There are three beautiful candles in the kitchen.
There are three beautiful candles on the table in the kitchen.
In this case, we are still only learning one new piece of information, but the answer contains more than a single word.