When do you use 'hay'?
Look at this example: - Hay un libro en las mesa. - El libro está en la mesa.
Why isn't this the same and when do you use 'hay'? What does 'hay' means actually?
All i know is this, but it's still kinda vague: - hay (existencia) - estar (localización)
4 Answers
Hay means 'there is' or 'there are'; referring only to the fact that it exists/they exist.
Hay dos libros. There are two books. Two books exist. This sentence works fine as it is or you can add further information, such as in your example:
Hay un libro en las mesa.
There is a book on the table. (focusing on the existence of the book)
está(n) 'it is' / 'they are'; referring not to their existence but to their present state (happy/sad/dirty/clean etc) or their present location. You will then need to add further information to complete the sentence.
El libro está en la mesa.
The book is on the table. (focusing on where the book is)
Dos libros están en la mesa. Two books (they) are on the table.
Hope that helps somewhat 
Hay basically means "there is/there are". It is also used in some idiomatic expressions. But for your purpose here, the first example would say:
There is a book on the table.
The second sentence would be:
The book is on the table.
Hello and welcome to the SpanishDict forum
Hay means = there is/ there are
eg: Hay tres coches en el aparcamiento = There are three cars in the car park
It is frequently used when talking about some weather conditions:
Hay luna = The moon is shining/There is moonlight o Hay neblina it is foggy
Hay is also used in the impersonal construction: ''hay que'' meaning someone should do something
eg: Hay que lavar los platos = Someone should wash the dishes
Hay que dar de comer al perro = Someone should feed the dog
Hay que limpiar la casa = Someone should clean the house
Our qualified online Spanish teacher: Paralee Whitmire also mentions this construction in one of her lessons.When I find the correct one I will add a link:link text
I hope this helps 
This is the same as for English: There is a book on..., The book is on....
The first sentence uses the indefinite article "a", so this can be any book. The second sentence uses a definite article, the, and is referring to a specific book. 
