1. It is common for a sentence to start with an introduction. An introduction can be anything from just one word to a long clause. In general, an introduction is used to state a time, a place, a condition, a frequency or a fact before the main part of the sentence. (Introductions vary hugely.)
Examples:
In the centre of London, the number of people who fell victim to pickpockets
rose by 30 per cent in a month.
(sets a place)
As soon as the cake is golden-brown, take it out of the oven.
(sets a time)
Yesterday, the manager visited the stables.
(sets a time)
On Tuesday 4th July a band played carols in the park for 8 hours.

("On Tuesday 4th July" sets a time. It is an introduction and should be
followed by a comma.)
Having spoken to John, I can confirm that the meeting is definitely off.
(states a fact)
As you are well aware, the latest figures do not look promising.
(states a fact)
After the secretary had read the minutes of the meeting, the chairman
asked for the financial report. 
A band played in the park for 8 hours on Tuesday 4th July.

On Tuesday 4th
July, a band played in the park for 8 hours .
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ONLY USE A COMMA FOR AN INTRODUCTION
Words that "set the scene" do not always start the sentence; they could appear at the back or in the middle. You only need a comma when these words appear at the front of the sentence. This is very handy to know. Look the examples below:
At 4 o'clock, the new manager, David Bain, will visit.
(correct, but too many commas)
The new manager, David Bain, will visit at 4 o'clock.
(correct - much tidier)
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