1. The precedes a noun or noun phrase that needs no further qualification—for example:
The weather was horrible.
The Senate is a mob of 100 wholly owned political subsidiaries. [USA Today]
[S]omeone reported being bitten by a dog. The dog was quarantined. [The Umpqua Post]
2. The precedes a thing that is about to be clarified. Here are three examples in one sentence (the fourth the being of the first type):
The earthquake that hit Haiti last month destroyed the house of the man who owns the world’s largest collection of Haitian art. [Guardian]
3. When a mass noun calls for an article, the is used: