the

Used when your listener will know which one you mean.

Example: Compare "I went into a room." and "I went into the kitchen."

Used with a stress, to show that the word following is special.

Example: Are you the John Smith that I went to school with?

Used with an adjective that acts like a noun to mean all of the people or things concerned.

Example: The poor need our help.

Used with superlatives forms of adjectives and adverbs.

Example: You are the best.

The is used before a comparative to make a pair with another comparative, showing that as one changes, the other changes (the X-er, the Y-er).

Example: The bigger, the better.