The word interjection comes from the Latin word inter (between) and jacĕre (to throw). So, an interjection is a word that you throw in between sentences or thoughts to express a sudden feeling. According to Meriam Webster Dictionary, an interjection is a word or phrase that is grammatically independent of the words around it, and mainly expresses feeling rather than meaning. 

         Though interjections are one of the traditional parts of speech, they are grammatically unrelated to any other part of a sentence. For example, the word “wow” or “ouch”, is solely designed to convey emotion in an abrupt and exclamatory way. Interjections are very common in spoken English, but they appear in written English as well. In writing, an interjection is typically followed by an exclamation point, but it can also be followed by a comma if it is part of a sentence.

         Interjections are commonly used at the beginning of the sentence. They’re also associated with a punctuation mark designed to convey emotion. 

e.g.

- “Oh no, I can’t believe that it is snowing here again!”

- “Wow, this is such a pleasant surprise!”

- “Oh my, I knew I should have turned off the oven!”

Since interjections depend heavily on how they are used in speech, the punctuation they take varies greatly, according to context.

         Interjections don’t always have to be at the beginning of a sentence. They can appear in the middle, in the end, or anywhere else the author wants to interject a bit of feeling and emotion.

e.g.

- “So, it’s snowing again, huh?”

The interjection “huh” is found at the end of the sentence and is designed to express confusion at the continued snow falling. 

            Because interjections usually express sudden feelings, you’ll often see them used to convey surprise or excitement. 

e.g.

- “Oh gosh! I can’t believe how late it is.”

- “Yikes! There’s a snake in the garage!”

The interjection “oh gosh” and “brr” is a standalone sentence with an exclamation mark. This is grammatically correct, even though those sentences don’t contain a subject or verb, both are normally required for a complete thought. The interjection, or the emotion, is the entire point of the sentence.

         There are literally hundreds of interjections in the English language. Most are designed to express strong emotions, such as love, hate, surprise, happiness, anger, enthusiasm, disgust, or unhappiness. Some interjections can express either a mild emotion or can be expressions, such as “Excuse me”.

         In an era of increasing communication via text messaging and social media which is often laced with interjections, experts say that paying more attention to these loud and forceful parts of speech will help create a better understanding of how human beings actually communicate.

Sources:

https://www.grammarly.com/blog/interjection/

https://grammar.yourdictionary.com/parts-of-speech/interjections/what-is-an-interjection.html

https://www.merriam-webster.com/dictionary/interjection#note-1

Nordquist, Richard. (2020, August 26). Definition and Examples of Interjections in English. Retrieved from https://www.thoughtco.com/what-is-an-interjection-1691178