Why one should not start a sentence with a conjunction?

Answer 1

One View

It may offend the stipulations of Latin prose composition to begin a sentence with a conjunction, but it is perfectly good English to do so. If the conjunction but can begin an independent clause it can begin a sentence.

Another View

With no Academy of American English, the idea of "proper grammar" can be elusive. To answer this question directly assumes the premise that one should indeed not begin a sentence with a conjunction.

The answer, assuming this premise, would be something like this: If your professor or boss recommends against it, then don't begin a sentence with a (coordinating) conjunction.

The question then becomes, "What is the grammatical foundation for the proscription?"

The reason for the proscription is to avoid sentence fragments. A sentence must be able to stand sensibly on its own.

If you look at the sentence, "But I stayed at home" we see a sentence that is not sensible on its own. If you consider the sentence, "I stayed at home" you will notice the sentence is not in context any more than the first example sentence, but it is a sensible sentence.

Many people will explain that beginning a sentence with a coordinating conjunction adds emphasis. This can be accomplished using an EM dash--but few people take advantage of the tools in their tool box. The exclamation mark is also a well know, if under utilized method of adding emphasis. The semicolon too deserves more attention than it receives when separating independent clauses that are closely related.

The next time you see a sentence beginning with a coordinating conjunction, ask yourself if it deserved emphasis. More often than not, the author simply doesn't seem comfortable writing a compound or complex sentence.

If you look around the web, you will likely find more sources saying it is OK than those that say it is not proper grammar. You may then decide it is much easier to side with the former. Remember that citation and style refer to usage, but questions of grammar are posed out of the need to understand the rules of grammar.

These rules would dictate that you not begin a sentence with a coordinating conjunction.

Answer 2

Starting a sentence with a conjunction is generally considered informal or conversational in style, so it may not be appropriate in all writing contexts, such as academic or formal settings. However, it can be used effectively to create a more casual or engaging tone in creative writing or informal pieces.