Commas act as a "pause for breath" in English and help give structure to sentences. Use commas to separate clauses from the rest of the sentence. Clauses which appear at the beginning of a sentence should be followed by a comma, for example: "When I phoned him, he read me his shopping list." If we were to reorder that sentence, the comma would no longer be necessary: "He read me his shopping list when I phoned him." These types of clauses are known as subordinate or dependent clauses and often begin with words such as "although", "while", "when" and "despite", among many other examples.
Sometimes clauses or phrases appear in the middle of a sentence and then a pair of bracketing commas is necessary, for instance: "The rain, which had poured incessantly since Monday, stopped without warning and the sun shone brilliantly." As you can see, the basic sentence would be "The rain stopped without warning and the sun shone brilliantly." By using a pair of bracketing commas, you can add extra information in a way that makes sense to your reader.
Practise your punctuation skills with this quiz on commas.