punctuation: colons and semi-colons, dashes and brackets
Chan Ming from Hong Kong writes:
  Please tell me how I can best use the semi-colon and the dash in writing compositions.
Roger Woodham replies:
Semi-colon or colon?
The colon is probably more useful to you in writing compositions, Chan, as the colon symbol : is used before an explanation about something or to introduce a list. Note these examples below:
As a student living in London I know all about money pressures: part-time jobs barely cover the cost of accommodation and there is no money left for going out.
Yet two of my internet applications for credit cards were accepted within five minutes of my logging on: a high-street bank offered me a student card with a ?00 monthly spending limit and a building society offered me one with a ?00 limit.
I am able to survive financially only by:
- not having more than three credit cards in operation at any one time
- paying off part of the debt each month
- not going out more than twice a week
Colons are also commonly used to introduce sub-divisions within a list:

There have been many winners from Britain's summer heat wave.
Seasides: the main resorts have drawn crowds of up to 250,000 a day.
Lidos: outdoor pools have had their best summer for ten years.
Vineyards: British wines will enjoy a vintage year and early harvests.
Farmers: cereal farmers have benefitted from an early harvest.
Supermarkets: barbeque sales are 800 per cent up this summer.

Note also that in a more formal English style colons are frequently used to introduce direct speech:

A spokesman for the British Chamber of Commerce remarked: 'This country is not very well set up for dealing with very hot weather: public transport has been particularly problematical this summer.'

Semi-colons - symbolised as ; - do not have as many uses as colons, but they are sometimes used instead of a full-stop to balance two sentences or clauses which are closely related in meaning:

Some people do their best work in the early morning; others perform best late at night.
Many people dislike walking in the rain and getting wet; personally I find it very therapeutic.

Dashes or brackets: - or ( ) ?

Dashes and brackets perform similar functions. They are useful for separating off parts of a sentence which introduce subordinate information which could be omitted. Dashes are used more in informal writing and newspaper reporting; brackets are more characteristic of discursive writing. Compare the following:

Two internet companies accepted my applications for credit cards immediately. Three further companies - Capital Two, Easyspend and Quickmoney - said they would notify me in ten days.
Otzi, as he is known, is by far the oldest mummy discovered - those from ancient Egypt are 1000 years younger - and was unearthed by a couple climbing in the high peaks of Italy's South Tyrol.
Dr Jones, head of international research, told the meeting - organised to express academic concern about the Atkins diet - that there wasn't a shred of evidence to suggest that the diet worked.
After high school I went to a provincial university far away from home (my parents were opposed as they wanted me close by) to study psychology.
There I met Amanda Fielding talking animatedly to a group of fellow students (they were media/film/TV studies undergraduates mainly, though some of them were linguists). I just knew immediately she was going to be a powerful influence on my life.

Dashes also introduce afterthoughts, particularly those of a surprising or unexpected nature. Brackets cannot be used in this way:

She says she won't be joining us at the summer house - at least not until after the children have left.
Everyone turned up for my twenty-first birthday - including Peter and Jane from Australia.