Many people were taught that they shouldn't end a sentence with a preposition. Today, however, most language experts don't abide by this "rule"—it's often called a myth. (Read more at Ending a Sentence with a Preposition.)
However, after I posted the article about ending sentences with prepositions, I was surprised by how many people asked if it is OK to start a sentence with a preposition. I've never heard a rule forbidding that practice.
Prepositional Phrases at the Beginning of a Sentence
Prepositional phrases at the beginning of sentences are common and grammatically correct. Consider these examples:
- On the other hand, Bobby likes strawberries.
- After soccer, we go out for pizza.
- By noon, all the runners should be finished.
- Over spring break, Shondra broke up with Lance.
Commas After Prepositional Phrases at the Beginning of a Sentence
When you start a sentence with a prepositional phrase, it's usually a good idea to put a comma after it (as in the examples above). In general, the longer the prepositional phrase, the more you need the comma. For example, the Purdue Online Writing Lab says a comma is required after introductory prepositional phrases that are longer than four words.