Hmmmm. Does that sentence need a comma? Purdue University's Online Writing Lab offers several helpful tips. 

#1: Independent clauses joined with a connecting word require commas.

Independent clauses are complete sentences that include a noun and verb. Sometimes you connect two independent clauses together to form one sentence by using a connecting word called a conjunction. Some common conjunctions are "and," "but," "for," "or," "nor," "so," "yet." 

Use a comma before the conjunction to separate the two independent clauses.  

• Example: We've worked hard, and our efforts have been noticed.

Do not use a comma before "and" if the words following it do not form a complete sentence.

• Example: I value nurses and admire their work.

#2: Two independent clauses joined together with no connecting word require a semicolon. 

When two independent clauses are joined together to form one sentence without a connecting word between them, use a semicolon between the two sentences.

• Example: I am going home; I intend to stay there.

#3: Two independent clauses joined together with a conjunctive adverb require a semicolon after the first sentence AND comma after the conjunctive adverb.

Use a semicolon when you join two independent clauses together with one of the following conjunctive adverbs: "however," "moreover," "therefore," "consequently," "otherwise," "nevertheless," "thus." 

• Example: It snowed all night; however, the roads were cleared by noon.

#4: Introductory phrases require commas.

Use a comma after an introductory phrase. Here are some common words used to begin introductory phrases: "after," "although," "as," "because," "if," "since," "when," "while."

• Example: After the webinar, we will answer questions.
• Example: While I wait, I'll watch this webinar.

#5: Introductory words require commas.

Use a comma after an introductory word, such as "yes," "however," or "well." 

• Example: Well, I didn't see that coming.
• Example: Yes, Extension has a wide range of webinars.

#6: Non-essential phrases require commas.

Non-essential phrases in the middle of a sentence are set off by commas. How do you know if a phrase is essential or non-essential? Purdue offers this test:

• If you leave out the phrase or word, does the sentence still make sense?
• Does the phrase or word interrupt the flow of words in the original sentence?
• If you move the phrase to a different position in the sentence, does the sentence still make sense?
  If you can answer yes to any of these, the phrase is non-essential and should have a comma before and after the phrase.
  • Example: The plant, which blooms in spring, is deadly to cattle. 
  • Example: The article, indeed, is stolen.

#7: Essential phrases should not be set off by commas. 

Other times, a phrase is essential to the meaning of the sentence. Do not use commas with essential phrases. Most often, phrases that begin with “that” are essential and should not be separated from the rest of the sentence by a comma. A good test: if you need all the words for the sentence to make sense, don't use a comma.  

• Example: She jumped out of the car while it was running. (While it was running changes the whole meaning of the sentence; thus, it's essential and not set off by a comma.)
• Example: The plant that grew in poor soil soon wilted. 

#8: Double adjectives describing the same noun require a comma between the adjectives. 

Coordinate adjectives are words that describe the same noun. If you can add the word "and" between the adjectives without changing the meaning of the sentence, use a comma between the adjectives. 

• Example: Illinois Extension provides relevant, science-based information that serves residents and businesses across the state. (Both relevant and science-based describe the word "information," so, a comma should be used between the words.)

#9: Yes, use an Oxford comma in a series: today, tomorrow, and forever. (See what I did there!) 

When listing a series of words, always use a comma before "and" or "or." We know this is a break from AP style, but it is the style preferred by campus; thus, it is Extension's preferred style. (Secret: it's my preferred style, too!)

• Example: Pets benefit our physical, mental, and social health in measurable ways. 

Read more at Purdue University's Extended Rules for Commas and Commas vs. Semicolons, the primary resources for today's 5-Minute Marketing. If you really love language, check out Conquering the Comma slide deck on the site.