It’s important to remember that introductory words and phrases—any words or phrases that come before the main clause in a sentence and often modify the subject of the sentence—should be set off with a comma. Here is an example of a sentence that does not include the required comma and then the correction:
Racing against the clock the university student rushed to submit her paper by the midnight deadline.
Racing against the clock, the university student rushed to submit her paper by the midnight deadline.
For more information on this comma rule, be sure to check out the Comma section in the Punctuation area of Grammar Essentials.