English literally has thousands of rules for punctuation and grammar that help to make sentences more easily understood. Here are a couple of key puctuation guidelines.
A COMMA USED IN A SERIES OF ITEMS
Of all the questions writers have, the one most asked is whether to place a comma before the last item in a series of three or more words or phrases. This is probably because the rule has actually changed over the years. As recently as the 1960s, one might have been told that the use of a comma would be "optional" in this case.
In today's world of professional business writing, however, if the last item is preceded by and, or, or nor, place a comma before the conjunction as well as between the other items.
Reference: The Gregg Reference Manual, 11th ed. (2011), para. 162.
NUMBER OF SPACES AT THE END OF A SENTENCE
As a general rule, use one space after a period (or other terminal mark) at the end of a sentence. Only use two if you feel a stronger visual break between sentences is needed. For certain, use only one space for text with justified margins or for manuscripts to be used for typesetting.
In the past, the rule for two spaces began primarily due to the quirks of the typewriter. Expert typists working at lightning speed on a keyboard found the keys would jam together with only one space after the period. That was because each arm of the machine had both capital and lower case letters on it. The "carriage control" shifted up or down to type upper or lower case letters. Putting only one space after punching the key for the period did not allow the arms to come to rest -- they jammed. With today's computers, we have "desktop publishing," and two spaces are not necessary.
Reference: The Gregg Reference Manual, 11th ed. (2011), para. 102.