The most common punctuation error made by academic writers is omitting a necessary comma.1 We have edited journal articles having text clean in every respect but this, leaving off those commas. This article covers three cases of missing comma. For when you should not include a comma, see That comma should not be there!.
1. Offset clause
An offset phrase should be marked off by commas (commas at both the start and the end). An offset clause is used here to mean a clause that is an aside, a clause that can be removed from the sentence and the sentence still reads correctly. In the following, the offset clause is highlighted:
✓ Rodríguez knew that, to pass the examination with flying colours, his language had to be flawless.
With this example, if you remove the highlighted clause, the sentence is still grammatically correct, that is,
✓ Rodríguez knew that his language had to be flawless.
Alternatively, use no commas at all (if the clause is short). This is readable (although some might argue that it needs commas):
✓ Rodríguez knew that to pass the examination his language had to be flawless.
On the other hand, if the clause is not an aside, do not use a comma.
✓ Rodríguez explained that the study was valid.
🗴 Rodríguez explained that, two in every one people globally are schizophrenic.
Most common mistake
The most common mistake with offset clauses is leaving off the first comma, as in:
🗴 Rodríguez knew that to pass the examination with flying colours, his language had to be flawless.
✓ Rodriguez knew that, to pass the examination with flying colours, his language had to be flawless.
Red flags
If you write “that although”, know that this probably signals an offset clause. The same goes for that if, that when, that while, that should, because although, and because if.
2. Introductory clause
An introductory clause does not form part of the main body of the sentence and thus should be marked off with a comma.
🗴 The following year Kahneman was awarded the Nobel prize.
✓ The following year, Kahneman was awarded the Nobel prize.
✓ On realising it was doable, Sophia registered for a Masters.
3. Comma missing before a terminating clause
This is the mirror image of the error with introductory clause. A clause at the end of your sentence that does not form part of the main body of the sentence should be offset with a comma.
✓ It is a common method used in social-science studies, according to Creswell and Creswell (2018).
✓ Ezra offers writer-specific recommendations on style, acronym use, and punctuation, among others.
More guidance
Grammarbook.com lists 15 cases in which a comma is needed.
Notes
- This is an informal observation, not a research finding.