Kaksoiskirjaimet, double letters

In Finnish almost any letter can either stand alone, as in kana, tulin.
 Or it can be doubled, like N in kanna, U and I in tuuliin.

The single letters and double letters are pronounced very differently. It's important to pronounce (and write) them right, because it changes the meaning of the word.

Tapaan sinut, means I will meet you, but:
Tapan sinut, means I will kill you. So be careful.

The basic rule is, that a single letter lasts for a normal amount of time, and a double letter lasts for double the time. 
That's why the double letters are called 'long' because they last for a longer time. And that's the only difference. The single a and double aa, are pronounced exactly the same, with the mouth and tongue in the exact same position, but the double aa just lasts for a longer time. Same with the single n and double nn.

With the letters P K and T, it's harder to keep the sound in your mouth for double the time. You do it so, that for example with double pp, you first close your mouth as if to start saying the letter p, but you stop there, for a while, with your mouth in the 'p' position, and then after waiting you finally let the p sound come out. With the single p there would be no waiting with your mouth closed.

When you say double tt, you start saying the letter t, and then stop with the tip of your tongue touching the top part of your mouth, stay there, wait for a while, (quiet) and then let the t sound come out.

Same with double kk. You stop right before making the k sound, and wait for a while.

Kivi stone, kiivi kiwi fruit
Tule come, tuulee it's windy
Katu street, kaatuu falls over
Värin of a color, väärin wrong
Tuli fire, tulli customs
Satua fairy tale, sattua to hurt
Laki law, lakki cap
Tipua chick, tippua to drop
Etc.

What about two different vowels after each other? Like AI, YÖ, IE, OU? The combinations of two letters are always pronounced like this: first you pronounce the sound of the first letter, then you pronounce the sound of the second letter. Those two sound after one another, is how you pronounce the two letter after one another.

Like this :
the AI, consists of A, which has the sound of A in father, and I, which has the sound of E in me.
So:
first pronounce the A(father)sound, after that, the I(me)sound: together you should have something that sounds like the word Eye in English.

So now we should be finished with the learning to read and write part. Just keep in mind the three basic rules of Finnish spelling:
1. A letter is always pronounced the same way, with one sound.
2. A combination of letters is pronounced so, that you first pronounce the sound of the first letter, then the sound of the next letter, then the sound of the next letter etc.
3. A double letter is pronounced like a single letter, except that it lasts double the time.