Commas will often set off a phrase at the beginning of the sentence from the subject and verb appearing later. There is not much to say on the introductory comma, but there are a few different scenarios in which you might need one.
Place a comma at the end of a prepositional phrase occurring at the beginning of the sentence. This comma can be eliminated IF the phrase is less than five words AND it doesn’t cause confusion.
EX: Beyond the sea, you’ll find your dreams.
One comma for what I considered linked prepositions.
EX: Under the boughs of the trees in the wind, I sit on a bench.
Multiple commas for AND prepositions.
EX: Over the hill, under the bridge, over the river, I found my dog.
This only works if the phrases are in SERIES. Over the hill and under the bridge and over the river, I found my dog.
Dependent clauses can actually be viewed as introductory phrases. For all the types above, there is no comma placed when it comes at the end of the sentence. Unless there is a large amount of contrast between the phrase and the sentence.
EX: You’ll find your dreams beyond the sea.