Hi Pedro
If you take a look at the last time I made some corrections to your writing, you'll see that I changed 'writen' to 'written', but I didn't mention it. I pointed out a couple of vocabulary and grammar errors, but I hoped that you'd notice my correction to 'written' and just kind of absorb the correct spelling.
It's actually not hard to work out why the past participle has two 't's : just think about the pronunciation. Remember that 'write', 'wrote' and 'writing' have a single 't' because these are long vowels, whereas 'written' has a short vowel, so it has a double consonant. Just think about 'kite' and 'kitten', for example - one 't' for the long /ai/ vowel in 'kite', and two 't's for the short /i/ vowel in 'kitten'.