What is the difference between the passive and active tense in Spanish and how do you form the passive?

The main difference between the active and passive is that in an active sentence, the agent (the person who does the action) is the subject of the verb. For example: Juan escribe un libro and los chicos hacen los pasteles con chocolate.

However, when we use the passive, the reciever becomes the subject instead of the doer. It can be formed mainly in two ways: 

1. Using ser + part participle (+por): el libro fue escrito por Juan.

It is important to remember adjectival agreement with this construction in terms of gender and number. 

2. Passive with se + 3rd person verb (singular or plural depending on the subject): Se hizieron estos pasteles con chocolate. 

It is important to remember that 'se' takes away the subject - it refers to 'them' so the subject is not needed. 

In these passive sentences, the doer of the action (the agent) is no longer the subject and instead it is the recipient.