mixed boring sentences again! :)
1) I will know the result of the examination by the end of the month.
2) I'll be going to Rome next Saturday. Correct
3) This time next week Juliet will be doing the exam. Correct
4) Will you lend me your computer tomorrow?
5) I go out with my friends most weekends. Correct
6) Mary goes shopping now and then. Correct
7) It's the first time Marc has ridden a bicycle. Correct
8) Simon has been watching television for hours. Correct
9) John has played guitar twice this week. Correct
10) We waited for the waitress who didn't arrive. Correct
11) We have been waiting for the waiter for an hour.Correct
Actually I don't think this kind of mechanical grammatical practice is so helpful.It might be better to write a "Non boring" piece that you actually enjoy! Just my professional opinion.