Open the brackets
1. If the lesson (be) more interesting, I would pay attention.2. David (not laugh) if you told him this joke.3. If he (finish) earlier, he will call me.4. If it (not rain) we would have had dinner outside, in the garden.5. I would have bought her a card if I (know) it was her birthday.6. We (replace) your computer if there’s something wrong with it.7. If he didn’t like this job, he (do) something else.8. If I (have) a bigger house, I might have more pets.9. If I had made the reservations, I (book) a better hotel.10. We will call you when he (come) back.

I know that we use were with I, but with he, she, it?... never heard about it

(We can use 'were' instead of 'was' with 'I' and 'he/she/it'. This is mostly done in formal writing). I found this... so I guess we can use both of them, but "were is better"... Thank you, I will know.

I mean it is so only in a SECOND Conditional sentence. To make your sentence look gramatically correct, when you have to write a second conditional sentence, you ought to use "were" instead of "was". In other kinds of sentences you use "was" and "were" where needed as normally.

ok, ok, I understood :) Thank you

You're welcome. =)