The biggest challenge in writing is to avoid boring the reader. One form of boring writing is repetitive sentence structure.

Consider this sentence: "He opened the bedroom door."

A good enough sentence, describing a simple action, moving the action forward in a straightforward fashion.

Now look at this paragraph:

   He opened the bedroom door. He stepped silently up to the bed. He
   pulled out a gun from under his jacket. He fired three shots into the
   sleeping body in front of him.

Even though this describes a dramatic action scene, it's just about as boring a piece of prose as you'll find anywhere. And yet, there's nothing wrong with any single sentence - the problem is repetition, of sentence structure and of the sentence subject: "He... He... He..."

So, how do we improve this? How can we vary the sentence structure and avoid the repetition of "he".

For starters, "he" has a name - use it part of the time. Not all of the time, of course. And use the name early - be careful of "He opened the door. Jim stepped into the room." since that sounds like these are two different people.

Next, we can re-order some of the sentences. For example: "Reaching under his jacket, he pulled out a gun."

We can also look for alternate subjects. In the example above, for example, we might be able to use: "The gun fired three times."

Each of those approaches can be overused, of course, leading to various forms of bad writing. Is the following an improvement on the first version?

   The door opened in front of him. The bed came closer as Jim stepped
   into the room. Jim's gun came out from under his jacket. The gun fired
   three shots into the sleeping body in front of Jim.

What we want is to be able to use a variety of techniques and structures, and to use them effectively. When is it best to use a longer, more complex, sentence, or a short and direct one? When can we use a different subject than the actual character performing the action ("Jim fired the gun" versus "The gun fired")?

The exercise: in 300 to 500 words, lead a character through a sequence of actions. Be careful not to get bogged down in unnecessary detail ("he turned the door handle, pushed the door open, stepped outside, closed the door behind him."), but don't omit any required steps either ("Oh no, Jim's been captured." "Thanks for freeing me," Jim said once he was freed.)

(You may also want to review Florence Cardinal's exercise on Transitions: http://www.internetwritingworkshop.org/pwarchive/pw15.shtml .)

When critiquing, look at how well the sentence structure was varied to keep things moving along without getting tedious. If necessary, make suggestions about how to improve flow and pacing.