articles

Articles are linked to and occur hefore the noun. Thus, it is: "an idea", "the idea", "the ideas".

The indefinite article (a, an) occurs before singular, countable nouns, but never occurs before plural, countable nouns -- "a student" but not "a students". In addition, the indefinite article does not occur before unmodified noncount nouns -- not "a air".

The definite article (the) may occur before singular countable nouns, plural countable nouns, and noncount nouns -- "the student", "the students", and "the air".

We use ‘a / an’ (for singular countable nouns) when we think the listener or reader
WON’T know which thing or person we are talking about. We use ‘the’ when we
think the person listening WILL know which thing or person we mean.

It’s not important if the person who is speaking knows which one. The important
thing is if the person who is listening knows or not. For example:

• I bought a blue sweater yesterday (the speaker knows which sweater, but the
listener doesn’t, so we use ‘a’).

• We went to a lovely café (the speaker knows which one but the listener
doesn’t, so we use ‘a’).

Sometimes the person who is speaking doesn’t know which one exactly (and neither
does the listener). Instead, the speaker is talking about any member of a certain group
– it doesn’t matter which one. We still use ‘a/an’, because the listener doesn’t know
which one. For example:

 I'’d like a cup of tea (the speaker doesn’t know which one, neither does the
listener. It doesn’t matter which particular cup of tea).

 I need to see a doctor (any doctor, neither the listener nor the speaker are
thinking about a particular doctor).

 Could you pass me a pen please? (any pen, the speaker doesn’t know which one).

The listener might know which one we mean because we’ve already talked about the thing in our conversation (or piece of writing).

 I bought an apple and an orange. The apple was delicious.

We use ‘an apple’ and ‘an orange’ at first because we think the person listening won’t
know which apple or which orange we are talking about.

But the second (or third or fourth…) time we talk about something, we can use ‘the’
because the listener knows which one. He or she knows because we’ve already said
which one – it’s the apple that I bought yesterday and not another apple.

Idea is a singular countable noun. I will suggest you an idea.

Ideas is a plural countable noun.  I will suggest you some ideas.

The idea you suggested me last night was not practicable. (Talking about the particular idea)