What are the general rules regarding the use of the definite article in French?

As with all French rules, there are exceptions to article usage. In general, however, the rules are as follows: 

Definite Article: (le/la/l'/les- in English-"the")

-The definite article should appear before a noun that has been previously identified in the phrase: "Quelqu’un a frappé à la porte." THE door is identifiable from context. 

• It should appear a noun that is determined by a noun complement phrase: "Le cadeau que tu m’as promis" THE present here is definite because it is the one you gave to me. 

• It should appear before parts of the body: "Tu te laves les mains"

• It should appear when a noun is specifically singled out: "Regarde la belle maison!"- The linguistic equivalent of pointing. 

• It should appear before universal nouns, known by all and easily identifiable: "La musique... Le soleil.. L'anglais etc."

• It should appear before the names of countrys, but NOT before cities: "Le Canada, la Paris"

• It should appear before functions and titles: "La reine Elisabeth, le Professeur"

-It should appear before the days of the week, when the phrase is making reference to a habit that is performed on that day. This is the French equivalent of saying "each Saturday, I drink coffee": "Le samedi, je bois du cafe"

• It should appear when making reference to a whole group of  the same noun. English would generally not use an article here, so this is one to bear in mind. If I hate mysoginistes, I would say in French: "Je déteste les mysogynes"

• It should appear before units of measure when a price is being described: "Cela coûte cinq euros le kilo"