Knowing when to use 'a' or 'the' can be difficult in English. Fortunately, there are some rules to help you.

First of all, you need to decide what type of noun you are using. Nouns can be countable, such as 'pen'. We can say 'one pen' or 'two pens'.

Rule 1

When you have a single, countable noun, you must always have some sort of article before it.

We cannot say "please pass me pen", we must say "please pass me the pen" or "please pass me a pen" or "please pass me your pen."

Nouns can also be uncountable. Uncountable nouns are either concept nouns, such as 'information', 'imagination', 'reliability' and so on, or materials and substances, such as 'water', 'money', or 'air'.

Rule 2

Uncountable nouns cannot have "a" in front. This is because you can't count them.

For example, ‘advice’ is an uncountable noun. You can't say "He gave me an advice", but you can say "he gave me some advice" or "he gave me a piece of advice".

Some nouns can be both countable and uncountable. For example, we say "coffee" meaning coffee in general, but we can say "a coffee please" when we ask for a cup of coffee.

Rule 3

You can use "the" to make general things specific. You can use "the" with any type of noun - plural or singular, countable or uncountable.

Please pass me a pen. (any pen)
Please pass me the pen. (the one that we can both see)
Children grow up quickly. (children in general)
The children I know grow up quickly. (not all children, just the ones I know)
Poetry can be beautiful. (poetry in general)
The poetry of Hopkins is beautiful. (I'm only talking about the poetry Hopkins wrote)