Y and EN
Although these two pronouns are also object pronouns they deserve a section of their own as they can be a bit more difficult to work out.
Like the pronouns presented above (le, lui, leur, etc.), they stand in place of a word or group of words to avoid repetition.
Note that 'y' very often has the meaning of 'there', as in the impersonal expression 'il y a' (there is/are). It can also be used as an indirect object pronoun when the indirect object pronoun refers to a thing rather than a person.
The pronouns 'y' and 'en' are used to replace an expression that would normally have been introduced by 'à' or 'de' respectively.
For example, in the sentence 'Il pense trop à son travail' (he thinks too much about his job), the part in bold could be replaced by 'y' while in the sentence 'Il parle trop de son travail' (he talks too much about his job), the part in bold could be replaced by 'en'. Notice the place of the pronouns: in both cases they will come before the verb (as it is the case for other object pronouns): 'Il y pense trop' and 'Il en parle trop'.