English contractions are very common in everyday speech. We use them to save time and to show we’re on informal, friendly terms with the person or people we are talking to.
Speech without contractions sounds very formal to us—maybe even stiff or “stuck up"-- as if the person speaking feels superior.
(However, contractions are uncommon, and often unacceptable, in formal writing like business proposals, essays, or term papers.)
English contractions can be hard to understand when you are just learning the language. In the U.S. you might hear two young people saying
“Dija see Sally at the game?”
“Nah, she must’ve bin sick—or she kuda gone out with Bill instead. Hey—gotta go now. Seeya soon!”
In correct written English, that would have been: “Did you see Sally at the game?” “No. She must have been sick, or she could have gone out with Bill instead. Hey, (I’ve) got to go now. (I’ll) see you soon!”
For more examples, see (and listen to) this short Voice of America talk on listening to and recognizing the grammar in everyday conversations.
This mock-rap video lists positive contractions. (It won’t show on a mobile phone—most screens are too small.) For his Negative Contraction list—in the form of a funny rap on a bad relationship-- see Negative Sentences.
You might have noticed that many contractions have two or more possible meanings: he's= he is or he has; she'd= she had or she would, etc. That doesn't bother English speakers because the meaning is usually clear (at least to us!) from the context.
Other contractions are easier. The examples below (first modals, then ‘be’ and the helping verbs ‘have’ and ‘do’) give the same expression without a contraction in parentheses.
Incidentally, we almost never say ‘Do you not?’ We would always ask: ‘don’t you…?’
For more examples, see Negarive Sentences, The Verb 'to Be' (be+ main verb ending in -ing for the present continuous tense), Modal Verbs (showing that 'would' is followed by the base form of the main verb) and Past & Present Perfect Tense Use (showing that the past perfect helping verb 'had' is followed by a past participle.)