In most cases, use the active voice of verbs instead of the passive voice. The passive voice moves the object of the verb to a superior position as the subject of the sentence, relegating the proper subject to an inferior role. Examples:
Jewelry is often stolen by burglars. [passive]
Burglars often steal jewelry. [active]
Passive forms often use the verb was:
Oxygen was discovered in 1774 by Joseph Priestly. [passive]
Joseph Priestly discovered oxygen in 1774. [active]
In the examples above, the proper subject—the doer of the action—often gets lost in the shuffle.
The active voice is almost invariably more vigorous, direct, and vivid and therefore keeps the action in sharper focus for the reader. You don’t have to change every passive construction to an active one. For instance, various stock locutions such as The project was abandoned and The Romans were defeated are perfectly acceptable.
Also, the passive voice is useful when the doer of the action is unknown or unimportant:
The lock was broken sometime after four o’clock. [Who broke the lock is unknown]
In 1899, a peace conference was held at The Hague. [This sentence comes from an essay by E.B. White. In this case, the doers of the action—the holders of the conference—are unimportant to White’s point.]