It’s customary to follow an introductory phrase with a comma unless it’s really short and doesn’t seem to need one. You’re never wrong to use one, so I would recommend taking the guesswork out of the whole ordeal by just using one there.
The most common introductory elements you’ll probably encounter are participial and prepositional phrases. Before you freak out at these grammatical terms, remember that each kind of phrase just represents its own simple part of speech.
Participles end in -ed or -ing and act as adjectives.
Wishing the children a happy summer, the teacher put them on the bus, waved goodbye and did a little happy dance.
“Wishing” is the present participle. The whole phrase acts as an adjective modifying “teacher.”
Flushed and feverish, he slept all day and most of the next.
“Flushed” is the past participle. The phrase acts as an adjective modifying “he.”
Write some sentences using each of the following participles in an introductory phrase. Be sure that the word that follows the phrase is the word that you mean to modify, like “teacher” and “he” in the examples above. Follow the models by separating the phrase from the main part of the sentence with a comma for clarity. Be sure that a complete sentence follows that comma.
Saving Attempting Attempting Terrified Honored
Meandering Juggling Pedaling Astonished Educated
(Look for my versions on the Adjectives Before a Noun post. If you’re feeling unsure, look at a couple of them first and then use them as models as you write your own.)
Prepositional phrases include a preposition, an article (usually) and a noun. In a sentence, a prepositional phrase acts as an adjective or an adverb. In general, prepositions set up relationships between things and other things and can fall anywhere is a sentence. When they appear as the introductory element, we usually separate them from the main sentence with a comma.
In the rafters of the barn, the pigeons nested all that spring. Jeesh! What a mess they made!!
Two for one! “In the rafters” tells us where the pigeons nested and so acts as an adverb. “Of the barn” tells us which rafters we mean and so acts as an adjective. There is no need for a comma between these two phrases because the second one modifies the first, but we do separate them from the main sentence.
At the beginning of the month of June in the twenty-third year of the twenty-fifth century, the people of the world were feeling pretty good about the state of the planet.
You can string together prepositional phrases forever. Here you don’t need a comma until you get to the main clause because each of the phrase modifies the one that comes before it. So they combine to make one big happy introductory element.
Write some sentences using at least ten of the following prepositions in introductory phrases. Follow the models by separating the phrase from the main part of the sentence with a comma for clarity. Be sure that a complete sentence follows that comma.
Here, in no particular order, are twenty-five common prepositions:
Up Down Under Around Through In
Out Around Over Between Across Along
For Beneath Against Within Beside Toward
At To After From Before With Until
(Look for my versions on the Adjectives Before a Noun post. Once again, if you’re feeling unsure about how to start, look at a couple of them first and then use them as models as you write your own.)