An interjection is a word or phrase used to express an emotion. Interjections are not grammatically related to the rest of the sentence in any way. They only serve to express emotion. Interjections may be punctuated with an appropriate ending punctuation mark or attached to a sentence with a comma.
Interjections made up of only one word are called primary interjections. Primary interjections do not belong to any other classes of words but only denote a sound associated with an emotion.
Ouch! I stubbed my toe.
Ouch is a primary interjection expressing the sound one makes when in sudden pain.
Ew, that bug looks so creepy.
Ew is also a primary interjection. It is just a sound used to express disgust.
Hmm, I wonder what’s for lunch.
Hmm is a primary interjection used to denote the sound made when thinking.
Interjections made up of words belonging to other classes of words are called secondary interjections. Secondary interjections may be made up of one word, a phrase, or a clause. Although the words making up a secondary interjection may be classifiable and grammatically relevant in other uses, as interjections they are still grammatically unrelated to the rest of the sentence.
What? I thought I would win!
What is a secondary interjection. It is a real word with other grammatical capabilities, but as an interjection it is used to express emotions such as confusion, surprise, or disappointment.
Goodness gracious! Be more careful next time!
The phrase goodness gracious is a secondary interjection. Both goodness and gracious can serve other grammatical functions on their own. However, as an interjection, the phrase is used to express emotions like surprise or worry.
Oh well, maybe we’ll do better next time.
Oh well is another secondary interjection. Although oh might be primary on its own, it is often used with other words to make phrases like oh my, oh no, and oh my goodness, which all contain words with other grammatical functions.