Sentence structure & punctuation · Topic 14 of 17

Commas

Tap any line to see why it matters

Grammar · commas

Commas separate series items, come before FANBOYS joining two sentences, follow an introduction, and wrap around information the sentence could lose.

01 · Two sentencesComma + FANBOYS joins two sentences

02 · IntroductionsComma after an introductory element

03 · That vs whichthat = no commas; which = commas

Commas between 3+ items in a series: gloves, gowns, and masks must know

Comma before FANBOYS joining two complete sentences: It rained, so we stayed in must know

Comma after an introductory word, phrase or clause: After lunch, we left must know

Commas around nonessential info, none around essential: My sister, a nurse, called must know

Comma between adjectives only if "and" fits: a long, tiring shift · a busy county clinic

Dates: comma after the day and after the year: On May 4, 2026, the unit opened

Comma between city and state, and after the state mid-sentence: Tulsa, Oklahoma, is home

Commas set off a direct address and interrupters: Please sit, Mr. Diaz

Serial (Oxford) comma = style choice, not an error: eggs, toast, and juice

that = essential, no commas · which = extra detail, with commas

✗ Dr. Okafor who runs the lab retired → ✓ Dr. Okafor, who runs the lab, retired

Commonly confused

	Essential (no commas)	Nonessential (commas)
Needed to know who or which?	Yes: it narrows the noun down	No: it adds an extra detail
Remove it and…	the meaning changes or gets vague	the meaning stays the same
Clause word for things	that	which
Example	The chart that is on the desk is new.	The new chart, which is on the desk, is ready.

Mid-sentence extra info needs 2 commas — ✗ Our coach, Ana arrived late

No comma when the 2nd part has no subject — ✗ She sat up, and ate

No comma between subject and verb — ✗ The nurse on nights, called

Removal test: meaning unchanged without it → nonessential → commas

Show 1 example sentence

that / which: The sample that came back positive was retested, but the second sample, which arrived later, was clear.

Show 3 more facts

Short introductory phrase: comma optional unless it prevents a misreading: In May we moved

Two very short clauses may drop the comma before and: I cook and she cleans

Commas around a degree after a name and before a "not" contrast: Lin Wu, RN, spoke

Pearl

Cross out the words between the commas: if the sentence still means the same thing, the commas belong there.

Practice this topic, then test the whole section.

HESI and HESI A2 are registered trademarks of Elsevier. NurseSavvy is not affiliated with or endorsed by Elsevier.