Is this a misplaced or dangling modifier? Emma Sue was delighted when Mr. Nguyen returned her perfect calculus test with an ear-to-ear grin.

This "Emma Sue was delighted when Mr Nguyen returned her perfect calculus test with an ear-to-ear grin" is a misplaced modifier.

Explanation:

A modifier does not describe the word next to it logically, so it is a misplaced modifier. It does not clearly mention the details in a sentence. This can be changed by modifying the sentence as follows: Mr Nguyen showed her correct calculus verification with an ear-to-ear grin, which delighted Emma Sue.

In the given, "with an ear-to-ear grin" denotes the prepositional phrase. It explains test if its target to be Mr Nguyen. In simple, termed as misplaced when the modifier presents after a word and not describes logically.