A transitional phrase (e.g., however, as a result, consequently) will usually start a new sentence. However, if you would like a smoother transition between the first sentence and the one starting with the transitional phrase, you can use a semicolon before the transitional phrase. You cannot use a comma.
Semicolon before a Transitional PhraseThis point is closely related to the lesson Extending a Sentence with a Semicolon as it concerns merging two sentences to form one. However, in these examples, there is a short bridge between the first half and the second half of the sentence. The bridge is known as a transitional phrase. There is always a comma after a transitional phrase but not before.
A transitional phrase will usually start a new sentence, but if you would like a smoother transition than that afforded by a full stop / period, you can use a semicolon before a transitional phrase to merge the new sentence with the previous one.
Examples (transitional phrases shown in bold):
START A NEW SENTENCE NINE TIMES OUT OF TEN
Transitional phrases are common. Most of the time, a transitional phrase will start a new sentence. However, you can use a semicolon if you wish a smoother transition. You should not do this too often.
DEFINITELY NOT A COMMA
You cannot merge two sentences with a comma. This is a very common mistake.
The error described above is called a comma fault or run-on error. This error is most commonly seen with the word however.
When the word so is used to mean therefore, it is a transitional phrase and should be followed by a comma.
Commas after a transitional phrase
Extend a sentence
Using semicolons before conjunctions (and, or, but, etc.)
Using semicolons in lists
Using semicolons to extend a sentence