According to Merriam Webster, an interjection can be defined as an ejaculatory utterance usually lacking grammatical connection. Interjections are words that are put in a sentence to convey an emotional state. For this reason, they are usually followed by an exclamation mark. It is uncommon to see an interjection in formal writing, unless it is in a direct quotation. Interjections are often found at the beginning of a sentence, however, there is no rule that says they have to be. They can also be used to signal that something should be obvious or to express excitement, surprise, or dismay.
The word “Duh!” is an interjection used to suggest that something should be obvious.
“Duh! You have to take it out of the package before you can use it.”
When expressing excitement or surprise, the following interjections can be used as demonstrated.
“Hey! How are you?”
“Oh! That is not good.”
“Oops! Didn’t mean for that to happen.”
“Yay! That is the best news I have heard all day.”
Interjections can also demonstrate surprise in the form of a question.
“What? Are you serious?”
“Huh? Is that what she said?”
In other cases, interjections are set off with a comma.
“Well, there is extra sugar in the cabinet.”
“Now, let’s see how we can fix this problem.”
“Here, let me get the door for you.”
In addition, interjections can occur in the middle of a sentence
“It’s so easy that anyone can use it–well, almost anyone.”
Used in informal writing, interjections can add spice to the message that you are trying to convey. However, their expressions can be very powerful, so try not to over use them. Too many can make your writing appear immature and too casual.