The experience of exercises

I have done the exercises on the Diane Hacker site, having a great time while doing that. I have to say there exercises help me a lot, some problems I did not solve before were settled. For example, when doing the E-ex 8-1 Active vs. passive verbs, I finally figure out the difference between active and passive. I was confused by this two frequently before doing the exercises. Although some exercises were easy for me, most of them were very helpful. Like the E-ex 11-2 Mixed constructions one which left me a deep impression, I got a very hard time while I doing this part. For instance, using worlds such as when, which, what, how in a mixed constructions. However, after finishing these exercises, I cannot say I work it out totally, getting a great improving it is. About e-ex 13-3 shifts, This exercise was helpful, although I already knew that the tense needs to be the same, it still remind me that the voice needs to be either direct or indirect and a person should not ever use the word "you" either.