Nice work! Some corrections:

1. I have some important reasons for not going to the party.
2. He said it so forcefully / assertively / passionately / confidently/
3. I can't find the definition of the word. [Perfect!]
4. The distribution of food must be fair.
5. Have I done something illegal? [Perfect!]
6. Sara gave me a private room [like a hotel] or Sara gave me one room [just one room]
7. I think, it is well concept of the origin of universes. [I don't understand what you're trying to say here]
8. I couldn't identify the thief. [Perfect!]
9. Life occurs [or exists] everywhere on the Earth.
10. For the last two weeks I have studied / I have been studying criminal law.
11. One hundred years ago many factories used only manual labor [-or in American English, -our in British English].
12. - I've forgotten you book. -  It's no big deal. / It's not important. / I don't care. / No biggie.
13. I don't have access to this/that section.

Did I write these sentenses corectly?

1. I have a reasonable reason not to go to the party.
2. He said it formidably.
3. I can't find the definition of the word.
4. The distribution of food must be fair.
5. Have i done something illegal?
6. Sara gave me an individual room.
7. I think, it is a good concept of the origin of the universe.
8. I couldn't identify the thief.
9. Life is present  everywhere on the Earth.
10. Last two weeks I have been studying criminal legislation.
11. One hundred years ago many factories used only manual labor. 
12. - I've forgotten your book. - Don't worry. It's insignificant.
13. I don't have access to the section.