Pastured rabbits will dig. Digging is part of their nature, any way. And as a result, if you keep rabbits on pasture then you need to make sure that you secure the pens so that they don't get out.
The best way to do this is to bury the outside fence to at least 1 foot down into the ground. However, even if you just do this, you will find that the rabbits can still get out if they are determined to do so.
So, in addition to this, you also need to place rocks around the outside the pen. This way, if the rabbits try to dig themselves out, they usually find that they can't get very far because the rocks are there.
Like any livestock, you have to visit your animals daily to check on fences, their welfare, etc. Rabbits are no different, but with this type of fencing treatment, you shouldn't loose your rabbits.
By the way, your English is fine! I learned German at High School for 5 years, and 30 years later can't remember 5 words! What you don't use, you loose, so keep using the language. :)