Why are these sentences grammatically incorrect?

I have all the answers to these sentences but I want to know why certain words need to be used instead of others.





1. The reason sharks do not have to eat as much as people think is because they are cold blooded, meaning they burn energy at a slower rate than their warm blooded counterparts.


It should be "as much as people think is THAT sharks are cold blooded"





2. Sometimes created by underwater earthquakes, tsunamis are the cause of considerable damage when they occur.


The "they" in the sentence should be changed but I dont know to what they should be changed to.





3. The amateur magician claims that his stunts are more impressive than the famous American magician and world record holder, David Blaine.


The "than" in the sentence should be changed but I dont know what it should be changed to.|||Most of these aren't because of "grammatical" errors, they just need a better sentence structure. In number 3. the sentence implies that the amateur magicians stunts are more impressive than David Blaine himself, not David Blaine's stunts.





It makes more sense written as:





"The amateur magician claims that his stunts are more impressive than American, David Blaine's world record holding stunts."





Though I'd probably use a synonym for "stunts" in an actual sentence, I used "stunts" twice to avoid confusion.





Hope that helps a little.|||Sharks eat much less food than the average individual would guess. This is because they are cold blooded, meaning they burn energy at a slower rate than their warm blooded counterparts.





Tsunamis, created by underwater earthquakes, are the cause of considerable damage.





I like three as it stands. You could say. The amateur magician claims his abilities are far superior to those of the famous American Magician and world record holder, David Blaine.|||2 looks fine to me so no help there (unless they mean to drop the whole "when they occur" because that's somewhat redundant), but 3 needs to say ...than those of the famous...