Equally important in understanding how to use commas effectively is knowing when not to use them. While this decision is sometimes a matter of personal taste, there are certain instances when you should definitely avoid a comma.

• Do not use a comma to separate the subject from its predicate:

[WRONG] Registering for our fitness programs before September 15, will save you thirty percent of the membership cost.

[RIGHT] Registering for our fitness programs before September 15 will save you thirty percent of the membership cost.

• Do not use a comma to separate a verb from its object or its subject complement, or a preposition from its object:

[WRONG] I hope to mail to you before Christmas, a current snapshot of my dog Benji.

She travelled around the world with, a small backpack, a bedroll, a pup tent and a camera.

[RIGHT] I hope to mail to you before Christmas a current snapshot of my dog Benji.

[RIGHT] She travelled around the world with a small backpack, a bedroll, a pup tent and a camera.

• Do not misuse a comma after a coordinating conjunction—this includes coordinating conjunctions starting sentences:

[WRONG] Sleet fell heavily on the tin roof but, the family was used to the noise and paid it no attention.

[RIGHT] Sleet fell heavily on the tin roof, but the family was used to the noise and paid it no attention.

[WRONG] But, we should try to do our best.

[RIGHT] But we should try to do our best.

[RIGHT] But, as you said, we should try to do our best.

• Do not use commas to set off words and short phrases (especially introductory ones) that are not parenthetical or that are very slightly so:

[WRONG] After dinner, we will play badminton.

[RIGHT] After dinner we will play badminton.

• Do not use commas to set off restrictive elements:

[WRONG] The fingers, on his left hand, are bigger than those on his right.

[RIGHT] The fingers on his left hand are bigger than those on his right.

• Do not use a comma before the first item or after the last item of a series:

[WRONG] The treasure chest contained, three wigs, some costume jewellery and five thousand dollars in Monopoly money.

[WRONG] You should practice your punches, kicks and foot sweeps, if you want to improve in the martial arts.

[RIGHT] The treasure chest contained three wigs, some costume jewellery and five thousand dollars in Monopoly money.

[RIGHT] You should practice your punches, kicks and foot sweeps if you want to improve in the martial arts.