Why is the sentence "She sighed, and began whispering again" grammatically incorrect?

Solution 1:

There's nothing wrong with that sentence.

Some would say that the comma is unnecessary so it should be removed. But it's certainly not wrong, and it could usefully indicate a pause between the sigh and the whisper. In any case, commas are punctuation, not grammar.

People who delight in pointing out others' grammar mistakes usually know less about grammar than they think.

Edit

There's further discussion of the purpose of the comma in the comments below, but here's a quote from Eats, Shoots & Leaves by Lynne Truss that explains it nicely:

“Thurber was asked by a correspondent: "Why did you have a comma in the sentence, 'After dinner, the men went into the living-room'?" And his answer was probably one of the loveliest things ever said about punctuation. "This particular comma," Thurber explained, "was Ross's way of giving the men time to push back their chairs and stand up.”

Hat tip to FumbleFingers.

Solution 2:

The comma in this sentence is perfectly fine.

If you were to take it out, you'd subtly change the meaning of the sentence:

She sighed and began whispering again

might imply that she sighed again and began whispering again. By inserting the comma, the writer makes it clear that only the whispering is the repeated act, that she hadn't necessarily sighed before.

It's a very minor distinction (and others may disagree). Comma usage is often subjective.

Solution 3:

The comma is the putative mistake. Typically, students are instructed not to put a comma between two coordinated predicates:

She_subject sighed_{predicate A} [ and began whispering again ]_{predicate B}

For many people, "grammar" is a collection of every language-related prohibition they've ever learned (or think they've learned). Presumably they've internalized the rule above, and are excited to catch someone breaking it. That is, after all, how you play the game of Grammar Gotcha.

Solution 4:

“When I saw him turning, I set my face towards home, and made the best use of my legs.”

-- Dickens, Great Expectations

“My proposal was to build a wharf there fit for us to stand upon, and I showed my comrades a large heap of stones, which were intended for a new house near the marsh, and which would very well suit our purpose.”

-- Franklin, Autobiography

It’s not difficult to find many, many more examples of this sort. If the OP is ungrammatical, it’s in good company!

(In other words, to be completely clear about it: it’s not.)

Solution 5:

Tina Blue writes sagely about Comma usage with Compound Predicates:

As with other compound sentence elements, a compound predicate generally is not separated by a comma. Occasionally, however, if the parts of a compound predicate are unusually long, or if the writer feels the need for special emphasis, a comma can be used with a compound predicate. Such commas should be treated as a heavy spice, though, and used sparingly.

So the rule appears to be 'you must not use this construction unless you want to'.

My sort of rule.

I might use it here: