The Passive, as in English is one of the three grammatical voices and is used to say that something is 'being done by somebody else'. In grammatical speak, we would say that the action that's described by the verb (eating, organising or whatever) is being done to the subject by an agent.
For instance: if we said in the active voice: 'the students are organising a party', in the passive voice, this would become 'a party is being organised by the students'. The verb with the action is organise, the subject is the party and the agent is the 'students'.
Just like with the active voice (where the subject of the verb actually does the action) it is available in all tenses - in the indicative and in the subjunctive.
A preposition is usually required and this is either par or de.
So, how does it work?
1) Move the direct object of the verb into the place of the subject.
2) Replace the verb form with the appropriate tense of etre + a past participle.
Now, this être must be in the same tense as the original main verb. Also, the past participle needs to agree in gender and number with the subject- like in any etre past tense verb.
Preposition wise: DE when we have a vague, indeterminate or habitual relationship- for instance: ....les legumes suivi de tiramisu
PAR when we have a specific, definite, or unusal relationship:
..Paul a été battu par Bob.
So, let's have a look at an example:
Les étudiants organisent une fête.
1) Move 'une fête' to the begining of the sentence.
2) The main verb, organisent' is in the present indicative, so we need to use the present indicative of etre in the passive sentence. In this case, we need 'est'
3) Agreements: fête is feminine so, our past participle must agree: 'organisée'
4) Preposition: par - this is a specific thing
Altogether: Une fête est organisée par les étudiants.
But, one important thing, Only the direct object of a verb can become a subject of a passive. INDIRECT OBJECTS CAN'T. Therefore, verbs that take an indirect object cannot be used in the passive voice.
So, for instance, 'demander x à quelqu'un' - this takes an indirect object, so the verb 'demander' can't be used in the passive voice.
Some other common examples include:
apprendre qc à qn: to teach somebody something
conseiller qc à qn: to advise somebody to do something
donner qc à qn: to give somebody something
But, ok, now we have seen the rules, why do we need it?
Basically just for stylistic purposes:
- To emphasise the sufferer of the action instead of the agent.
-As an alternative to 'on' when we don't know who did something- such as in police reports.
-Certain verbs + de to express descriptions: Le panier etait rempli de fraises.
- Certain verbs + de to express feelings, attitudes, and states of knowledge. Eg: Elle étatit connue de tout le quartier (we use de here because 'quartier' is quite vague and we don't know exactly who knows her).
But, when we have one of those verbs which cannot be used in the Passive, (verbs which are followed by an indirect object), it is important to be aware that it can be avoided.
There are many ways to avoid the passive, but, we shall focus on two for the moment and of those the safest by far is by using ON:
eg: On demande à Bob de partir.
Se voir + an Infinitive is also an option: Il s'est vu refuser: he was refused
To sum up, don't worry about the passive, just remember to make your past participles agree and don't use it with indirect objects!