Copyright basics - Insubstantial/Substantial
The Copyright Act (s.3) protects substantial parts of works as well as whole works.
Since "substantial" is not defined in the Act, the quantity and importance of what is being copied must be evaluated. In deciding whether a part of a work is considered substantial, the whole work must be taken into account. A few sentences from a novel would probably be considered insubstantial but a single line from a poem might be essential to the work and be considered substantial.