A conjunction is "a word such as 'and', 'but', 'while', or 'although' that connects words, phrases, and clauses in a sentence" (Cambridge University Press, n.d., para.1). For example: The cat and the dog are playing outside.
For an extensive explanation of conjunctions, including information regarding coordinating, subordinating, and correlative conjunctions, please visit the OWL at Purdue's Conjunctions and Coordination.
Cambridge University Press. (n.d.). Conjunction. Cambridge dictionary. Retrieved February 24, 2021, from https://dictionary.cambridge.org/dictionary/english/conjunction
Coordinating conjunctions serve to join clauses within a sentence. There are seven coordinating conjunctions: and, nor, but, so, for, or, and yet.
For example: I like vanilla cake, but chocolate is my favourite.
Tip: When writing for an academic audience, avoid starting a sentence with a conjunction. Since conjunctions serve to join two independent clauses, they should not be used to start a sentence.
For more information regarding how to punctuate independent clauses joined by a coordinating conjunction, please see the Commas, Semicolons, and Colons video.
The grammatical function of a coordinating conjuction is to connect words, phrases, and clauses. The problem with beginning a sentence with a coordinating conjunction is that doing so often leads writers to produce a sentence fragment. Strictly speaking, it's acceptable to start a sentence with a coordinating conjunction; however, I usually advise people to stay away from doing so in formal writing because the construction often adds an informal narrative tone that may not fit in with the expected tone of the document. My advice is that when you’re tempted to begin a sentence with a coordinating conjunction, consider the following: a) remember that the word should join words, phrases, and clauses; b) make sure that you won’t create a sentence fragment by beginning the sentence with the coordinating conjunction; and c) ensure that the sentence maintains the tone that is appropriate for the document. If you need help with finding another way to start the sentence, consider a transitional device instead.