How Good Is Your English Grammar?

What’s wrong with the following sentences?

1. You mustn’t stay in the building after 5pm if you don’t want to.
2. I work in my business since 10 years.
3. I have finished working on my website last week.
4. Could you please control the figures before signing the contract?
5. If I would have known, I would have told you.

Why not send me your corrections? To: cb@english-trainer.de And I’ll get back to you as soon as I can.