When do we use 'A', 'An' or 'The'?

We have always been told that we use 'a' or 'an' to refer to a single item, for example - a ball (one ball), a cat (one cat), an umbrella, etc.  Teacher also said we use 'the' to refer to an item when there is only one of it.  Confused?

We use 'a' to refer to one of many items.  There are many cats.  There are many mice.  I want to ask if cats chase one mouse.  Any cat.  Any mouse.  Not referring to one particular one, nor am I pointing to one.  So I say: "Does a cat chase a mouse?"

New scene.  I visit my friend.  He owns a cat and a mouse (the mouse has to be caged up to be out of reach of the cat!).  I ask if his cat chases his mouse.  I point to the items and ask: "Does the cat chase the mouse?"

See the difference?  We use 'the' to point to that person / thing / animal / place (noun) that we are talking about.  That is why we use 'a' or 'an' when we refer to something for the first time, and we use 'the' when we talk about it subsequently.  For example: We have a dog as a pet.  The dog we have is red in colour and its name is 'Clifford'.  OR  I have a fat, red pencil.  The pencil is so fat that it cannot pass through the slits of the drain when I dropped it onto the drain cover.

We use 'the' to talk about a thing that it is the only one on Eath.  For example:  The sun rises from the east.  (There is only one sun and one east)

Some countries, seas and groups of islands/mountains use 'the' in its name too.  A classic example is the United States of America or the Philippines, the Atlantic Ocean, the Himalayas, etc.

Confusion looms for some people when we have 'a' or 'an'.  For ease of teaching, teachers tell you "use 'an' for most cases in front of a vowel word".  They are not exactly wrong.  Only that it is not a 'vowel word', but a 'vowel sounding word'.  We say 'an umbrella', 'an usual story', but 'a university' and 'a European'.  Notice these last two words are pronounced with a 'U' sound.  Not how it sounds in 'umbrella'.  When in doubt, I will always sound the word and remember the example of 'university' and 'umbrella'.  It will use 'a' or 'an' according to which word its start sounds like.

'An' honest person (silent 'H') rather than 'a'.  'An' hour instead of 'a' (silent 'H' again).  Check against examples you can remember.  You should get the hang of it.

Now, the test for today:

Use 'a' or 'an' in the examples below:

a.  ____ hospital

b.  ____ piece of advice

c.  ____ advice

d.  ____ honest beggar

e.  "____ apple ___ day keeps the doctor away" goes the saying.  However, the one that you just ate will send you to hospital as it contains ____ worm!