Interjections
Interjections are words or sounds that are "thrown out" in speech. They are infrequently written, but people use them all the time when speaking.
Interjections serve no grammatical purpose, they just express emotion, so they are usually written separately (not part of another sentence). A lot of swearing is done through interjections.
How to use interjections in details
Because interjections stand alone from the rest of the sentence, you have to remember capitalization and punctuation! For example, with the sentence
I dropped my phone.
We can add the interjection Shit! to express our feelings about what the sentence describes.
Shit! I dropped my phone.
Interjections can be followed by exclamation marks, question marks, or periods. When interjections are placed in the middle of a sentence, they are set off by either commas or double hyphens (see below).
Placing an interjection in a sentence
The last example used an interjection at the beginnning of a sentence, but they can also be placed at the end of a sentence:
Mom is cooking spiders for dinner. Gross!
Sometimes they can even be used in the middle of a sentence:
My brother said —so stupid— that bats only eat cheese.
Not all interjections are one word, not all lone words are interjections
Although some interjections —"Uh oh! Oh no! No way!"— are made up of more than one word, most interjections are just one word. Ha! But not every word that gets used on its own is an interjection. Consider the following sequence:
Wow! Karen! Look!
Only the first word is an interjection. The second is a noun, using "direct address" (calling someone by their name). The last is a verb in the imperative mood (a command).
Interjection magic: swearing without swearing
Interjections get their power from how they are said, not from the word itself. This allows us to express strong feelings without using words that might offend people. For example, my grandmother used to say Oh sugar! instead of Shit!, and people often say Fudge! instead of another, less polite "fu" word. When a word is used this way, it's called a euphemism, which just means that one word is being used in place of another one.