In Spanish there are many ways to make impersonal sentences, for example using the impersonal "se": En este bar se come muy bien, or using impersonal verbs that never have a subject, like Llueve mucho. Using a passive form would also make a sentence impersonal, e.g El hombre fue acusado de homicidio. There is no hidden subject in any of these sentences.
Another way is to use the 3rd person plural form of the verb (the ellos/ellas form) in any tense, but without using the subject pronoun.
Have a look at the following examples:
Notice how in all the examples above there is no mention of "who" or "what" does the action; this is because we do not know or it is not relevant.
Important: This structure looks identical to using a 3rd person plural while omitting the subject pronoun "ellos/ellas". In impersonal sentences, we deliberately omit the pronoun because we simply do not know who or what performed the action of the verb.
This is not the same as when we choose not to use the subject pronoun because it has been mentioned previously and because the ending of the verb tells us who the subject is. This type of sentence is not an impersonal structure.
Be careful, you may wrongly infer all 3rd person plural sentences which omit the subject pronoun as impersonal. Or you may wrongly infer that all 3rd person plural sentences which omit the subject pronoun are just assuming you know who the subject is already.
Take a look at the first sentence above but this time with a subject.
Carmen y Rosa llaman a la puerta.
Carmen and Rosa are knocking on the door.
We know who they are because we were expecting them or we can see them through the peephole.
Llaman a la puerta.
Someone is knocking on the door/There's someone at the door.
We don't know who it is as we weren't expecting anyone in particular, it could be anyone.
Take a look at this other example:
¡Parad a esos dos hombres! ¡Me han robado!
Stop those two men! They robbed me!
Here we know who the subject is because it has been mentioned before, so ¡Me han robado! is simply omitting the subject because it is obvious and there is no need to repeat it. This is not an impersonal sentence.
¡Me han robado!
I've been robbed!
You may or may not know who robbed you, but even if you did know, the focus of this sentence is on what happened, not who did it. It is not relevant at that moment.
Take a look at these other examples:
- Expressing instructions and general statements with the impersonal se = one
- Using se debe/se puede to say You must/you can (impersonal sentences)