8.23 using non-finite clauses

Instead of using a finite time clause, you can often use a non-finite clause.

For example, you can say 'I often read a book when travelling by train', meaning 'I often read a book when I am travelling by train', and you can say 'When finished, the building will be opened by the Prince of Wales', meaning 'When it is finished, the building will be opened by the Prince of Wales'.

Note that you can only use a non-finite clause when it does not need to have a new subject, that is, when it is about the same thing as the main clause.