You usually follow an interjection with an exclamation mark. Interjections are uncommon in formal academic prose, except in direct quotations.
The highlighted words in the following sentences are interjections:
- Ouch, that hurt!
- Oh no, I forgot that the exam was today.
- Hey! Put that down!
- I heard one guy say to another guy, "He has a new car, eh?"
- I don't know about you but, good lord, I think taxes are too high!