Rhetoric and Composition/Missing comma after introductory element< Rhetoric and Composition
When you begin a sentence with a word or group of words that provides some background, introductory, or otherwise preliminary information, put a comma between this word or phrase and the rest of your sentence. The comma here tells your reader to pause, take the background information into consideration, and get ready to move on to the main part of the sentence.
To help you recognize places in your sentences where you are missing commas after introductory clauses, read your writing out loud. Chances are good you'll naturally pause after introductory phrases. You can also check the beginning of each sentence to look for words or phrases that add information about time, place, or manner or for words that serve as transitions; these are all common introductory elements.
Incorrect: Before the budget passed several lawmakers filibustered to stop it.
Correct: Before the budget passed, several lawmakers filibustered to stop it.
Incorrect: However supporters saw the legislation through.
Correct: However, supporters saw the legislation through.
Learn more under "Commas after introductory elements" here.