Which are the differences in the grammatical formation between the passive and the active voice in Spanish?

The active voice uses the simplest structure: Subject+Verb+Direct Complement. We can recognise the direct complement because it can be substituted by the "lo, la" pronouns. Example: Juan está cantando una canción= Juan la está cantando. The passive voice is the inversion of the active voice and uses the following structure: Direct Complement (which now is called passive subject)+Verb in passive voice+POR preposition+Subject (which is now know agent complement) We have two important aspects here: -The verb in passive is formation is the same than in English: verb to be in the correct tense (determined by the sentence in passive) and the participle of the main verb -POR preposition: which is always before the agent complement. Example: La canción esta siendo cantada por Juan.