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.