The pronoun y
Summary
The pronoun y replaces a place expression or a phrase introduced by à referring to a thing; it sits before the verb and is never used for a person.
Explanation
The little word y has two closely related jobs. First, it replaces a place: any expression of location introduced by a preposition such as à, dans, sur, en or chez can be replaced by y meaning 'there'. Tu vas à la bibliothèque? — Oui, j'y vais (yes, I'm going there). It saves you from repeating the whole location phrase.
Second, y replaces à + a thing with verbs that are construed with the preposition à: penser à (to think about), répondre à (to answer), réfléchir à (to reflect on), s'intéresser à (to be interested in), participer à (to take part in). Tu penses à ton examen? — J'y pense (I'm thinking about it). Here y stands for the whole à-phrase referring to a thing or an idea, not a place.
Position works like the other object pronouns: y goes immediately before the conjugated verb (j'y vais, nous y pensons), or before the infinitive when there is one (je vais y aller, on doit y réfléchir). In affirmative commands it follows the verb with a hyphen, and -er verbs keep their -s for sound: vas-y! (go on!), penses-y (think about it).
The crucial limit is that y never replaces à + a person. For people you do not use y; you either use an indirect-object pronoun (lui, leur) with verbs that take a person as indirect object, or, with verbs like penser à and tenir à, you keep the preposition and use a stressed pronoun: je pense à lui (I'm thinking about him), not « j'y pense » when a person is meant. So 'I'm thinking about it (the project)' = j'y pense, but 'I'm thinking about her' = je pense à elle.
A practical rule of thumb: if you can replace the phrase with English 'there', or if the verb is built with à and refers to a thing/idea, use y and put it before the verb. If à introduces a person, stop — switch to lui/leur or à + stressed pronoun instead. Also memorise the two ultra-common fixed expressions il y a (there is / there are) and on y va / vas-y (let's go / go on).
Examples
Tu vas à la bibliothèque ? Oui, j'y vais maintenant. — Are you going to the library? Yes, I'm going there now. (y replaces the place 'à la bibliothèque' and means 'there'.)
Tu penses à ton examen ? J'y pense tout le temps. — Are you thinking about your exam? I think about it all the time. (y stands for 'à + a thing'; for a person you would say 'je pense à lui'.)
Je vais y réfléchir ce soir. — I'll think about it tonight. (With an infinitive, y goes before the infinitive réfléchir.)
Vas-y ! — Go on! / Go ahead! (Affirmative command: y follows the verb and va keeps an -s (vas-y).)
Il y a trois livres sur la table. — There are three books on the table. (Fixed expression il y a = 'there is / there are'.)
Je pense à elle, pas à mon travail. — I'm thinking about her, not about my work. (For a person, keep à + stressed pronoun (à elle); y is not allowed here.)
Common mistakes
Tu penses à ta sœur ? Oui, j'y pense.
Tu penses à ta sœur ? Oui, je pense à elle.
The pronoun y replaces à + a thing or a place, never à + a person. With penser à referring to a person, you keep the preposition and use a stressed pronoun: « je pense à elle » (or à lui / à eux). Saying « j'y pense » about a person is wrong — y is reserved for things and places (j'y pense = I'm thinking about it, e.g. the project). Compare « Tu vas à Paris ? J'y vais » (place, y is correct).
Prerequisites
Related rules
pronounsword-orderprepositions