A/An = one of many possibilities. 1) use it the first time you mention something a [I need a chair] (it doesn't matter which chair, any will do) b [Yesterday, I bought a chair] (I haven't told you enough about this chair for you to imagine it yet, so it still is 'a' chair) 2) use it to talk about 1 thing *in general* in a formal settings / It's the same as using a plural form --- Since this is a generalization, it really means 'most' / 'almost all' c [A chair is something that people usually sit on] <--formal d [Chairs are things that people usually sit on] <-- less formal The - used to talk about a specific object 1) use it the *second* time you talk about something, your listener/reader already knows what you're talking about [umm... the chair is broken] (talking about the chair in example a] [The chair is so comfortable!] (talking about the purchase yesterday - describing the chair that was bought) 2) use it to talk about a unique object or something everyone knows [The moon is full tonight] - When we say 'The moon' it always means Earth's moon. [The president said....] - it is understood from your conversation which president you're talking about (usually your own country) 3) to describe ALL things in a general category [The chair has many forms and has been used all over the world] (this means *all* chairs) There are also special rules for names and geography.

We use "the" (= definite article) when we are referring to a definite object. We use "a" (= indefinite article) when we are referring to an indefinite object. Who destroyed the flowers in my garden? It was the dog. the dog → my neighbour's dog, or my dog, … the flowers → the flowers I planted there or the flowers I know growing in my garden Did you see that? Was it a ghost? a ghost → some ghost I don't know, I haven't seen it before A man (I haven't seen him before) is walking down the street. The man (walking down the street) has got a dog.

I'll put it this way: a=any the=this/that or these/those