It, this and that in paragraphs

We use it, this and that to introduce further information about a topic already mentioned. However, the words have different uses.

We use it to continue to refer to the topic we are already writing or speaking about:

The heart is the central organ in our bodies. It is used to pump oxygen around the body through the bloodstream. (It refers back to The heart)
The new album by ‘The Noughts’ went on sale yesterday. It is their third album in three years and is set to become as great a success as previous releases.

warning

We don’t use it when we first give information about a topic, for example immediately after a chapter or section title in a text:

(b) Green application form
This must be signed by all applicants and returned by 30 November 2009.
Not: ~~It must be signed~~ …

We can use this to refer back to whole clauses and sentences and to previous parts of a text. This highlights the information referred to much more strongly than it. Writers often use this when a point or idea is to become an important part of the discussion that follows:

More and more people are discovering that Tai Chi is one of the most valuable forms of exercise. This has led to a big demand for classes. (This refers back to a whole sentence.)
Heavy rains and stormy conditions throughout the summer have led to severe shortages in strawberries and other soft fruits. This has led to price rises in many supermarkets and shops.

We use that in a similar way to this. However, when we use that, we distance ourselves more from the topic or from aspects of the topic:

For many traditional football supporters, it is a problem that so many young girls and women attend football matches these days. That is a sexist attitude of course.

That is also used to refer to ideas associated with another person:

The chairman apologised for the poor performance of the company and promised a better future for investors. That was a promise many people felt he could not possibly keep.