You may copy portions from copyright protected material to illustrate a point that you are making in an assignment, a scholarly work, article, or blog posting (to name just a few contexts) without the permission of the copyright owner. You must always cite the source of any works you use. 

In many cases, small amounts of copyright protected materials are used in assignments and considered “insubstantial” - meaning it does not create a copyright issue. In other cases, you may be able to rely on fair dealing to support using larger portions without permission.

The amount used should be for the purpose of illustrating your larger point and would not normally involve copying an entire work. There may be a case where a significant portion of a work or an entire work must be used, such as with a photograph. As long as the context of your use supports the amount used, there is a strong case for it being fair.