Interjections, also known as exclamations, are words or phrases used to suddenly and briefly express strong feelings—for example:
What the heck?
As in these examples, interjections are usually treated as standalone sentences, even though they obviously don’t have subjects and predicates. In other cases, particularly in casual speech or writing, an interjection may be affixed with a comma to the beginning of a sentence—for example:
Hey, what was that?
Oh, I don’t know.
Interjections are also sometimes used in the middles of sentences. In such a case, set apart the interjection with commas—for example:
We’ve had some success and, thank god, a little bit of growth.
I’d say it’s, oh, about six miles.
She asked me to come upstairs for, ahem, a cup of tea.