Determining when to insert a comma before a phrase or clause that begins with because requires thoughtful analysis. In this post, I’ll walk you through two kinds of examples: sentences with positive verbs and sentences with negative verbs. The explanations and examples are derived from Claire Cook’s Line by Line (115–16).

Sentences with Positive Verbs

When the main clause of a sentence contains a positive verb, inserting a comma before because makes what follows nonessential to the meaning of the sentence:

Alex ordered the book online. Robert also ordered the book online, because he was running out of reading material.

In the sentence above, because he was running out of reading material is not essential to the meaning of the sentence. The point is that Robert, like Alex, ordered the book online. The reason Robert ordered the book online is simply additional information. But if the reason is necessary to convey the writer’s meaning, then the comma should be omitted:

Alex ordered the book online because she was too tired to go to the bookstore. Robert ordered the book online because he was running out of reading material.

In this case, the writer wishes to emphasize the reason each person ordered the book online.

Sentences with Negative Verbs

When the main clause of a sentence contains a negative verb, the insertion of a comma before because lets the reader know “the reason for a negative statement” (Cook 116):

I didn’t order the book online, because I was running out of reading material. I decided to buy the book from the shop down the street instead.

In the first sentence above, the reason the writer didn’t order the book online is because the writer was running out of reading material. As the second sentence indicates, the writer decided it would be quicker to get the book from a shop. 

The omission of a comma indicates that what follows because “gives an incorrect explanation” (Cook 116):

I didn’t order the book online because I was running out of reading material.

Here, running out of reading material is not the reason the writer ordered the book online, as the second sentence below makes clear: 

I didn’t order the book online because I was running out of reading material. I ordered it online to support Bookshop, a new online store.

But without that second sentence, the meaning of the first sentence is ambiguous. Did the writer order the book online because the writer was running out of reading material? Or did the writer order the book online for some other reason? Thus, as always when a sentence is unclear, it’s best to revise. In this case, you might rewrite the sentence as follows:

It was not a shortage of reading material that led me to order the book online. I ordered it online to support Bookshop, a new online store.

Work Cited

Cook, Claire Kehrwald. Line by Line: How to Edit Your Own Writing. Houghton Mifflin Harcourt, 1985.

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Jennifer Rappaport

Jennifer Rappaport is managing editor of MLA style resources at the Modern Language Association. She received a BA in English and French from Vassar College and an MA in comparative literature from New York University, where she taught expository writing. Before coming to the MLA, she worked as an acquisitions editor at Oxford University Press and as a freelance copyeditor and translator for commercial and academic publishers.