Final answer:
Signal phrases are used to establish the boundary between a student writer's ideas and the ideas from a source.
Step-by-step explanation:
Signal phrases should be used with sources because they establish where the student writer's ideas end and the source's ideas begin. These phrases indicate when information from a source is being used and help attribute ideas to their original authors. By using signal phrases, students can demonstrate proper understanding and respect for the ideas and work of others.
Learn more about the purpose and importance of using signal phrases with sources