le pronom y

uses

y expressing place

Y replaces a preposition indicating location plus its object: 'à UIC,’ ‘à Chicago,’ ‘chez moi,’ 'devant la Tour UH' (in front of the UH Tower). It is usually translated as there. Prepositions which indicate location include à, en, dans, devant, derrière, sous, sur. The noun objects of these prepositions are places and things which indicate a place. The exception is the preposition chez which is used  with a person, as in 'chez Marie' (Marie's place). 

NOTE:  à  +  person is replaced by an indirect object pronoun  (lui, leur)except with certain verbs. 

y with certain verbs

With certain verbs y  replaces the preposition à  when its object is an idea  or thing,  but not a person. Some of these verbs are penser à, réfléchir à (to think about), s'intéresser à  (to be interested in), répondre à  (to answer to), participer à  (to participate in). In these expressions, the preposition à is idiomatic. It does not indicate location in or movement toward.

When these verbs are followed by a person, the disjunctive pronoun will be used, for example, “Je pense souvent à mon ami.  Je pense souvent à lui.”  (I often think of my friend. I often think about him.) 

placement

Placement of y is the same as that of direct and indirect pronoun objects: y comes before the verb it refers to, except in the affirmative  imperative. 

In compound tenses, like the futur proche and passé composé, it comes before the auxiliary. 

NOTE:  There is no agreement between y and the past participle.