This is altogether different from the fact that Spanish subject pronouns are rarely required, but always implicit. If there isn’t an implicit subject, the sentence is impersonal.
Some verbs are impersonal (llover -> to rain) and others can act sometimes as impersonal verbs. You will quickly understand this on seeing the different kinds of impersonal sentences.
The most important types of impersonal sentences are the following:
1. With weather verbs.
They are only used in the 3rd person singular. These verbs are: llover (to rain), nevar (to snow), granizar (to hail), tronar (to thunder)…
Llovió toda la noche. It rained all night.
Mañana va a nevar. Tomorrow it’s going to snow.
2. With weather or time expressions with the verbs hacer, estar, ser.
With the 3rd person singular of these verbs. Hacer (to do/to make) is usually translated as “to be”. Estar and ser mean “to be”. They are used in the following constructions:
Hacer + noun
Hace frío/sol/calor. It’s cold/sunny/hot.
Hace buen/mal tiempo. The weather is good/bad.
Hace un año que no veo a Laura. It’s been a year since the last time I saw Laura.
Juan se fue hace horas. Juan left some hours ago.
Estar + adjective
Está lluvioso/nublado/soleado. It’s rainy/cloudy/sunny.
Ser + (de) + noun
Es de día/noche. It’s day/night.
Es lunes. It’s Monday.
3. With the verb haber.
The verb haber (to have) can act as impersonal in the 3rd person singular. The present verb form is special, hay. It is usually translated as “there is/are”, but remember that in Spanish it is impersonal, so there is no agreement. It is always in the singular.
Hay muchos niños en la clase. There are many children in the classroom.
Hubo un ruido muy fuerte. There was a very loud noise.
4. With the pronoun se.
They are formed with the pronoun se + verb in the 3rd person singular. They are widely used for general statements. Most of these sentences can be translated into English using “you”.
En este restaurante se come muy bien. You eat very well in this restaurant.
Se llega allí en solo cinco minutos. You arrive there in only five minutes.
Se habla demasiado. People talk too much.
Be careful not to confound these sentences with the reflexive passive voice.
5. With a verb in 3rd person plural.
Most of the se impersonal sentences can be changed into this kind. The pronoun se is omitted and the verb changes into the 3rd person plural.
Comen muy bien en este restaurante. You eat very well in this restaurant.
Llaman al timbre. Someone is ringing at the door.