Confirm that the actions described take place in the same time period. Look at the following sentence, which shows tense markers in italics:
While Lee was answering the doorbell, the intruder had entered through the bedroom window.
"While" signals that the action in the second clause of the sentence occurs at the same time as the action in the first clause. Yet the words "had entered" indicates that the intruder entered before Lee's action. The sentence shifts from the past progressive tense—action in the past that is in process—to the past perfect tense—action that occurred prior to another action in the past.
Revision: While Lee was answering the doorbell, the intruder was entering through the bedroom window.
Let's take another example:
Naming the five best movies of last year was easy. Ninety percent of the movies I see are lousy, and that leaves only a handful that are even worth considering.
Here, the sentence unnecessarily shifts from the simple past tense ("Naming . . . was easy") to simple present ("the movies I see") to present progressive ("are even worth considering").
Revision: Naming the five best movies of last year was easy. Ninety percent of the movies I saw were lousy, and that left only a handful that were even worth considering.
If the actions occur in the same time period and both are either in process or simple, make sure that the tense forms of verbs and verb phrases remain consistent. In the following sentence, an activity that takes place in the same time period takes different tenses because that activity is described in different ways:
Maria is always singing, and she sings beautifully.
This sentence is correct. Here, the time period remains the present, but the action described shifts from the process of singing (present progressive tense) to the simple act of singing (simple present).
