Problems with the Passive

False Passive

Grammatically, the false passive is the same as sein + an adjective.  This construction describes a condition rather than an action.  Das Haus ist verkauft is the false passive, while das Haus wird verkauft is the true passive.  The false passive sentence indicates that the house is already sold (condition), while the true passive indicates the house is in the process of being sold (action).

Passive with Absentee Subjects
Passive forms may have a definite or indefinite subject, or no apparent subject at all.  The accusative object of an active sentence becomes the nominative subject of the passive sentence.  But sometimes there is no accusative object.  Since a verb cannot be in the first position of sentence without turning the sentence into a question, es is used as the subject.  

Man antwortet ihnen nicht is an active sentence, but if it were turned into the passive, there would be no accusative object.  The passive would have to be es wird ihnen nicht geantwortet.  (Here werden agrees with the apparent subject, es.)

But if another element, such as a dative object or time expression, can be put in the first position, then es is omitted.  Ihnen wird nicht geantwortet can also be used as the passive.  There is no apparent subject, only an implied es, so the form of werden remains wird to agree with es.

Avoiding the Passive

1.  The construction man + an active verb can be used instead of the passive voice.  Man translates to one, you, we, they, people and constitutes the subject.

Diese Bluse wird gereinigt.  This blouse is being dry-cleaned
Man reinigt diese Bluse. They are dry-cleaning this blouse.

Der Dieb wurde gefunden. The thief was caught
Man fand den Dieb. They caught the thief.

2.  Man + modal + an infinitive is frequently used with müssen or können.

Der Flecken kann nicht entfernt werden.  The stain cannot be removed.
Den Flecken kann man nicht entfernen.  We can't remove the stain.

3.  Sein + zu + an infinitive can be used with können or müssen to express the possibility or necessity of an action.

Das kann schnell gemacht werden.  That can be done quickly.
Das ist schnell zu machen.  That is quickly done.

4.  Sich lassen + an infinitive can replace können and a passive infinitive.

Das kann gemacht werden.  That can be done.
Das läßt sich machen.  That can be done.

Showing Purpose

Weil (because) + a dependent clause shows the reason for an action; however, damit and um.zu (so that, in order to) show the goal of an action. Damit is also followed by a dependent clause, whereas um.zu introduces an infinitive.

Sie macht das Fenster zu, damit sie nicht friert. = Sie macht das Fenster zu, um nicht zu frieren.
She closes the window, so that she won't freeze . = She closes the window, in order to not freeze.

Commonly, you use damit when the subject of the main clause is different from the subject of the dependent clause, and um.zu when the understood subject of the infinitive is the same as the subject of the main clause.