Specific periods

Names of decades, centuries and historic periods, which refer to only one particular period, have the definite article, for example 'the nineteen-eighties', 'the nineteenth century', 'the iron age'.

They are an example of unique reference.

...the trend emerged in the sixties.

'Past', 'present', and 'future' generally have the definite article.

...the dangers in thinking only of the present.

...plan for the future.

But 'present' and 'future' can be used after 'at' and 'in' respectively with no article.

...since there is no certain answer at present.

Try to remember it in future.

In American English 'in the future' is used rather than 'in future'. It is possible to use an indefinite article when talking about the life of one particular person: 'He has a future', 'a man with a past'.

Illnesses

Article usage with the names of illnesses and other conditions is sometimes inconsistent, and can vary with the same word. Normally nouns referring to illnesses are uncount and do not have an indefinite or definite article.

...evidence that they caused cancer.

Here is a list of common words like this: AIDS, diarrhoea, malaria, tuberculosis, anaemia, hepatitis, pneumonia, typhoid, appendicitis, herpes, rabies, yellow fever, cancer, influenza, rheumatism, cholera, laryngitis, smallpox, diabetes, leukaemia, tonsillitis.

'Cancer' can also be count, but combinations with it are uncount, for example 'lung cancer'.

With the names of some common infectious diseases the definite article can be used, as well as no article, but it is not as common.

This applies to 'flu' (but not 'influenza'), 'measles', 'mumps', and 'chickenpox'.

She's coming down with the flu.

I had a mild attack of flu.

...the first symptoms of measles.

The names of less specific conditions, such as 'cold', 'chill', or 'cough', are treated as simple count nouns.

...when someone has a cold.

Words ending in '-ache' behave in different ways, in British English. 'Earache', 'toothache', 'backache', 'stomach-ache', and so on can be uncount or count, so you can say 'I've got earache' and 'I've got an earache'.

He was suffering from severe earache.

One morning she developed an earache.

...various infusions which she used for sore eyes, toothache and muscular pains.

...when a woman with a toothache was brought to us.

'Headache', however, is a count noun, and so you can have 'a headache' or regular 'headaches', but you cannot say 'I've got headache'.

Next morning she complained of a headache.

In American English, all '-ache' words are count nouns, so it is not possible to say 'I've got earache', and so on.

Meals

You can refer to meals without using an article when you are talking in general about the standard meals of the day.

Tim had dinner in the hotel.

Breakfast was already waiting for her.

If you are talking about individual meals, you can use the nouns 'breakfast', 'lunch', and so on as count nouns with an appropriate article. When used like this, the nouns are usually qualified or modified; you do not normally say 'I had a breakfast.'

... a hot breakfast.

...after a relaxed breakfast.

'Lunch' and 'dinner' can also be used alone with the indefinite article to mean a special formal occasion.

Afterwards, Her Royal Highness attended a dinner at the Castle Hotel.

It would be unusual to say 'I've been invited to a breakfast' since breakfast is not usually a formal occasion.