When combining two sentences into one compound sentence, a comma and a coordinating conjunction (and, but, or, nor, for, so, yet) are used.
Example 1: The festival was to be held today, but it was canceled because of the rainy weather. (comma and coordinating conjunction)
Sometimes writers use words such as however, furthermore, and therefore (these are called conjunctive adverbs) in place of coordinating conjunctions to combine two sentences into one. This is where a punctuation problem often arises. The mistake writers make is to incorrectly place a comma in place of a semicolon before conjunctive adverbs, as illustrated in the sentence below:
Example 2: The festival was to be held today, however, it was canceled due to the rainy weather. (comma preceding the conjunctive adverb – INCORRECT. Note: The comma following the conjunctive adverb is perfectly correct and should be left as is.)
A semicolon, rather than a comma, should precede conjunctive adverbs when they link two complete sentences. Note the corrected form of Example 2 below:
Example 3: The festival was to be held today; however, it was canceled due to the rainy weather. (semicolon preceding the conjunctive adverb – CORRECT)
Don’t be misled–there ARE situations in which it is entirely correct to use a comma before a conjunctive adverb, as in the following sentence:
Example 4: The sporting events, however, continued despite the weather.
Note that in this sentence, in contrast to the sentence in Example 3, no semicolon is needed because it is not a compound sentence.