Which sentence correctly uses a nonrestrictive phrase?

• A. The movie which we watched last night was long.
• B. The movie, which we watched last night was long.
• C. The movie which we watched last night, was long.
• D. The movie, which we watched last night, was long.

Answer: (D)

Nonrestrictive information adds extra detail about a noun and is set off by commas, because it can be removed without changing who or what is being talked about. Here, the phrase “which we watched last night” adds extra information about the movie but isn’t needed to identify it. That’s why it’s correctly punctuated with a pair of commas: The movie, which we watched last night, was long. Removing the clause leaves a complete, sensible sentence: The movie was long.

The other variants mix up this punctuation or use the phrase in a way that makes it seem essential. Without the second comma, or by treating the clause as restrictive, the sentence would imply you’re specifying a particular movie among others, which isn’t the intended nonessential detail.