Although pronominal adverbs exist in English as well, they are not as important as in Italian. But what are pronominal adverbs? Let's start with an example. The famous sentence from Ludwig Wittgenstein uses, in the English translation, two of them.
Whereof one cannot speak, thereof one must be silent.
Whereof and thereof are pronominal adverbs. They have characteristics of an adverb and characteristics of a pronoun. They resemble adverbs, because they never change, it doesn't matter what they respresent, a feminine or masculine noun or an idea, they never change their form. But at the same time they stand for something, represent something, in this case all the things one cannot speak about, so they do what pronouns do. The adverbial charakter is perhaps not really obvious in English, because nouns do not have a gender in English, so it is quite irrelevant if a pronomial adverb matches in gender and number with the nouns it stands for, but it is important in Italian. Pronominal adverbs are rarely used in English, normally they must be translated with the preposition 'it'. The preposition to use depends on the preposition required by the verb.
to think about
Do you think about the car ? Yes, I think about it.
to lie on
Do you lie on the laptop? Yes, I am lying on it.
If you speak French you know them already because they exist in French as well: en (ne), y (ci). The problem is that there is nothing equivalent in English and therefore they are complicated for English native speakers. As a general rule, not very reliable, we are going to see are more reliable one below, you can say that the translation of ne is some of it and ci must be translated with there, of it, to it, about it .
ci = there, of it, to it
Your parents live in Berlin? Yes, they live there. I tuoi genitori abitano a Berlino? Sì, ci abitano.
You often think of doing something else? Yes, I think of it very often. Pensi molto spesso a fare un' altra cosa? No, non ci penso molto spesso.
Are you accustomed to this kind of living? Yes, I am accustomed to it. Sei abituato a questo stile di vita? Sì, ci sono abituato.
Do you go to Rome? Yes, I go there. Vai molto spesso a Roma? No, non ci vado molto spesso.
Pay attention to what he says! Yes, I pay attention to it. Fa attenzione a ciò che dice! Sì, ci faccio attenzione.
The other way to explain the use of ci / ne, much more reliable, is based on the preposition required by the verb. If the verb requires a, in, su or per the correct pronominal adverb is ci.
ci (a, in, su, per)
pensare a qualche cosa = to think of
You thought of buying something to drink? Yes, I thought of it. Hai pensato a comprare qualcosa da bere? Si, ci ho pensato.
riuscire a fare qualche cosa =to succeed in
Have you succeeded in finishing the work? Yes succeeded in doing it. Sei riuscito a finire questo lavoro? Si, ci sono riuscito.
credere in = to believe in
Everybody believes on the project. Everybody believes in. Tutti credono in questo progetto. Tutti ci credono.
volare su = to fly over
The birds are flying over the town. They are flying above it.
Gli uccelli volano sulla città. Ci volano
ci without preposition
In some cases ci can be used as well if the verb doesn' t requires any preposition at all.
I can' t see any interesting in it. Non ci vedo niente di interessante.