English has a hiding place here and Italian does not. “There is” contracts down to “there’s”, and once it has contracted it stops counting: “there’s two people outside” is a sentence nobody objects to, and the number has quietly stopped mattering. Bring that habit into Italian and it comes out as c’è molte persone, which your ear has no reason to flag, because in the language the habit came from nothing was wrong. C’è and ci sono are two forms of one verb, and the number is audible in both.
C’è is ci plus è, so there is a verb in there, and it agrees with the thing that follows it, because that thing is its subject. One of something: c’è. More than one: ci sono. The number lives in the noun after the verb and nowhere else, which means the decision is made after you have chosen that noun, not before. English lets you commit to “there’s” and then discover how many there were. Italian asks you to know first.
C’è un problema.
Ci sono molte persone.
C’era poca gente.
Ci sono stati due incidenti.
Ci sono io.
The last two figures show how far the agreement reaches. Ci sono stati due incidenti has the auxiliary plural because the accidents are, and then the participle plural for the same reason, so one noun has set the ending of two words that came before it. And ci sono io is the same machinery taken to its limit: the thing that follows the verb is a person, that person is io, and the verb goes where io sends it. Nothing in the construction is fixed except the ci.
Every tense keeps the same split, so there is nothing new to learn once the pattern is in place. In the imperfetto it is c’era against c’erano: c’era un bar all’angolo, c’erano due bar all’angolo. In the future it is ci sarà against ci saranno. In the passato prossimo the auxiliary carries the number and the participle follows it, which gives c’è stato un problema and ci sono stati dei problemi, and with a feminine noun c’è stata una discussione and ci sono state due discussioni. Four endings and one idea. Whatever comes after the verb is the subject, and the verb belongs to it.
The edge that trips people up
Ci vuole is the same agreement one step further on, and it catches people who have the first part right. What it means is that something is needed, and the thing needed is the subject. The ci in it stands in for nothing, and the post on ci and ne takes that side of it. One hour: ci vuole un’ora. Two hours: ci vogliono due ore. The English is “it takes”, with an “it” that never changes, so there is nothing in the translation to remind you that the Italian verb is still counting.
Ci vuole un’ora, non ci vogliono due ore.
Then there is ci sta, which learners meet and reach for as a replacement for c’è. It is a real expression with other work to do. Nella valigia ci sta tutto says everything fits in the suitcase, and ci sto as an answer to a proposal says you are in. Neither of those is the neutral way to say that something exists in a place, which is what c’è does.
Nella valigia ci sta tutto.
How to hear it
Read your sentence to the end before you commit to the verb. The word that decides between c’è and ci sono always comes after it, and that ordering is the reason the English habit survives so long in Italian: you have said the verb while the noun is still a vague quantity in your head. Nothing later in the sentence corrects you, because by then the verb is spoken.
The repair is to build the phrase silently in the other order. Noun first, then the verb it needs, then speak. Due incidenti, so ci sono stati. Poca gente, so c’era. It feels backwards at first and then it does not, because the noun is the part you actually want to say and the verb was only ever agreeing with it.
Two nouns will still catch you after that, and they are the ones where the English goes plural and the Italian does not: gente and polizia. English says people are and the police are, so c’era gente and c’è la polizia both read as mistakes in translation and are right in Italian. Reading the sentence to the end is no help with these, because what has to be settled is which Italian word you are using, not how many people you are picturing. Gente and polizia are two words to know rather than two agreements to hear, and that is a different kind of knowing from everything else on this page.