The passive is a verb form which is especially used in written texts. The passive is used to highlight the object and the subject may not even be mentioned at all: 

The cake was baked.

The passive is formed of the auxiliary verb be and the 3rd form of the main verb. The negative form can be created by adding not in the sentence. The auxiliary verb be is conjugated according to the tense:

Her fatigue was caused by a vitamin D deficiency.

The subject of the passive sentence is the object of the main clause in the active sentence. The passive form can only be formed of transitive verbs that require an object: 

Proper sleep hygiene is to be followed to alleviate insomnia.

An auxiliary verb (e.g. can, could, may, might, should etc.) can be added to a passive sentence before the verb be. In this case, the predicate is formed of an auxiliary verb + be + the 3rd form of the main verb:

Many diseases can be prevented by leading a healthy lifestyle.

The passive highlights the object of the sentence but the subject can be expressed if necessary with an agent (by + agent) which is placed at the end of the sentence:

The meditation was lead by an experienced teacher.

The -ing form of the passive that takes place in the present is formed of the verb be + being + the 3rd form of the main verb. When the passive takes place in the past, the passive structure is having been + the 3rd form of the main verb:

Lauren is being examined by the doctor.

Elizabeth, having been infected, was quarantined for a few days.