Arnold is a lifeguard in the summer. He teaches swimming during the school year. [Use subordination.]
Arnold, a lifeguard in the summer, teaches swimming during the school year.
Arnold is a lifeguard in the summer and teaches swimming during the school year.
The first sentence has a subordinate phrase, a lifeguard in the summer. The second sentence does not use subordination.
For more help, see sections 14a and 14b.