Commas after a transitional phrase

A comma is used after an introduction that links the sentence to the previous one.

1. It is common for a sentence to start with an introduction that acts like a bridge between the last sentence and the new sentence. The introduction makes the transition between the two sentences smoother. (It is known as a "transitional phrase".) It always appears at the start of a sentence and is followed by a comma.

Examples:

Bruce Leonard spent 4 years in Japan studying Kung Fu and 3 years at the
London School of Martial Arts as the senior instructor. As a result, his style...

("As a result" is a transitional phrase. It acts like a bridge between the previous
sentence and the new one. It is followed by a comma.)

Of course, he never knew…

("Of course," < transitional phrase)

In summary, 7 out of 10…

("In summary," < transitional phrase)

Therefore, the cost of each lesson…

("Therefore," < transitional phrase)

Consequently, his teaching techniques…

("Consequently," < transitional phrase)

However, Bruce was unaware…

("However," < transitional phrase)

SEMICOLONS

On occasion, you may wish to use a semicolon before a transitional phrase to make the transition between sentences even more seamless. (This is covered in lesson "Semicolons".)

I cannot come on Tuesday. However, Peter will be there.

(However - transitional phrase)

I cannot come on Tuesday; however, Peter will be there.

(however - transitional phrase using a semicolon)

NOT A COMMA

You cannot use a comma before a transitional phrase.

I cannot come on Tuesday, however, Peter will be there.

This is a very common error.

Associated lessons:

Using semicolons before transitional phrases (e.g. however)
Using semicolons to extend a sentence
Commas after a sentence introductions
Commas after interjections (yes, no, indeed)
Commas before conjunctions (and, or, but)
Commas for parenthesis
Commas in lists
Commas with a long subject
Commas with numbers
Commas with quotation (speech) marks
Commas with the vocative case