Using Commas with Intro Words

Some sentences start with intro words.

During summer I love to fly kites.

       ("During summer" introduces the sentence.)

When you have four or more intro words, place a comma after them.

On a warm summer day, I love to fly kites.

       (A comma follows "On a warm summer day.")

Readers are looking for the main part of the sentence, with a subject + verb pattern. The comma signals that the main part of the sentence is starting.