“A/an” and “The” are the two types of English articles. There are rules to help you decide which one to use, but first, you need to know what type of noun you are using.
Grammar rule 1
When you have a single, countable English noun, you must always have an 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 in English can also be uncountable. Uncountable nouns can be concepts, such as ‘life’, ‘happiness’ and so on, or materials and substances, such as ‘coffee’, or ‘wood’.
Grammar rule 2
Uncountable nouns don’t use ‘a’ or ‘an’. 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 the product, but we say “a coffee” when asking for one cup of coffee.
Grammar 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.