answer:Don’t open the box if the seal has been broken. Is basically the same meaning, either can be used in this case; has been or is. The second sentence is more tricky. The sentence if you have enjoyed a good time is a little awkward. In English we would usually not put together enjoyed and good time, because it is redundant. We would say, if you enjoyed yourself at ABC, ring the bell. Or, if you had a good time at ABC, ring the bell. But, I caution you that if you are studying English with a teacher who also speaks English as a second language it is possible they are not aware of subtle difference. So, your two sentences would both be understood as the same, as you wrote them, but it is not how English speaking people would commonly phrase it. We would not write your mentioned issue has been resolved this afternoon. Once you add when it was resolved then using the verb was is more appropriate. in fact your mentioned issue is odd as well. It should be the issue you mentioned was resolved this afternoon. What you wrote would be understood, but not commonly used. If you leave off the time it was finished you can use has. The issue you mentioned has been resolved, is correct also. I am American, there might be subtle differences with British English possibly.