You should cite when:
- Referring to a source and stating someone else's opinions, thoughts, ideas, or research
- Using an image or media file that you did not create
When referring to a source, you have 3 options:
- Directly Quoting
- Summarizing
- Paraphrase
Which option you should choose depends on how much of a source you are using, how you are using it, and what kind of paper you are writing--since different fields use sources in different ways.
You do not need to cite a source for:
- Your thoughts and your interpretations
- Common knowledge