How to use articles (2): the

1 As mentioned in prior post, we use the before a noun when our reader and writer know which particular person, thing they are talking about.
I need the dress.(The listener knows which one)
I need a dress.(Any dress)

Here "the listener knows which one" usual means that (1) we have mentioned it before, (2) we say which one we mean, (3) it is clear from the situation which one we mean.
Examples are as follow:
 (1) He's got two kind of animals: a cat and a dog. The cat is black and the dog is brown. 
 (2) Who is the girl over there?
 (3) Andy is in the kitchen.

2 If there is only one, so the reader knows which one we mean. e.g. the sun, the moon

3 superlatives. This is because there is usually only one best, biggest etc. individual or group. So for the same reason, 'the' is used before next, last, first,same and only.

4 meaning 'the well-known'. After a name, an identifying expression with the is often used to make it clear that the person referred to is 'the well-known one'.
      She talk to Davie, the actor.

5 Do not use the with possessives and demonstratives.
      This is my daughter.(not the my daughter)
      Is that Ann's cat?(not the Ann's cat)
      I mean this dog.(not the this dog)

6 Usually do not use the with single proper nouns.
      Jeff lives in Toronto.(not The Jeff lives in the Toronto.)

7 Usually use no article to talk about things in general, 'the' does not mean 'all'.
       Books are expensive.(not The books are expensive.)

Also the pronunciation is slightly different several situations. 'The' is normally pronounced /Di:/before a vowel and /DE/before a consonant, which is dependent on its pronunciation other than spelling.

  the hour /Di: auE(r)/
  the MP/DI: em'pi:/
  the one-pound coin/DE wVn' paund 'kOIn/
  the university/DE' ju:nI'vEsEti/

Sometimes pronounce a stressed/Di:/ before a hesitation, or when we want to stress the following word, even if it begins with a consonant.
    He's the/Di:/- just a moment- deputy assistant vice-president.