How do I use 'y' in French?

One of the more daunting aspects of the French language is its use of monosyllabic words which can change the sense of an expression. Perhaps the most prevalent of these are 'y'. Often used to replace more complex concepts, it is all too easy to get confused as to when to use 'y' . Here are some rules of thumb to help alleviate the confusion. 

Y is usually used to replace a concept of place. Take the following phrase: 'Dans mon cours de chinois, il y a une fille aux yeux verts'. The 'y' here, part of the more complex expression 'il y a', meaning 'there is' - which, by the way, always requires the word 'y'!- replaces the moe complex idea of a place, namely the cours de chinois. This is most easily seen when answering a question. Consider the following dialogue: 

-Vas-tu au restaurant demain soir?

-Oui, j'y vais.

When answering the question, the 'y' becomes a replacement for 'au restaurant'. The expression 'j'y vais' meaning either 'I am going there' or 'I will be going there' should not be confused with the reflexive verb 's'en aller', meaning to leave, rather than to go anywhere in particular. 

The word 'y' can also replace a noun or pronoun, so long as it is preceded by the prepositions 'à, au, à la,à l', and aux'. Whether or not the improper noun is preceded by these prepositions is informed by the verb used. The two most common ones are 'penser à' and 'réflechir à'.

-Penses-tu toujours à lui?

-Oui, j'y pense.