People studying English often ask me What do you think of my accent? A lot of times, their pronunciation is quite good, but they just don't have a natural rhythm and intonation... and a lot of times, that's because they don't use contractions; either they've been told that they're incorrect or even vulgar, or they're just not in the habit of using them. But the thing is, contractions give English — both spoken and written — its natural rhythm, and incorporating them in your speech is one thing you can do right away that'll improve your accent by leaps and bounds. A mastery of contractions is essential to speaking natural English, since they're used extensively by all native English speakers (not just Americans).

What's a contraction?

A contraction is a combination of two or more words written and pronounced as one word. For example, the contraction we're, which combines the pronoun we and the verb form are. Contractions are always written with an apostrophe that generally stands for the part of the contracted word that's been omitted. 

Unfortunately, the pronunciation of contractions isn't always evident from the spelling. Some of them have more than one reading. For example, didn't is a contraction of did not and can be read [dɪ́ɖɪnt̚] or [dɪ́d̚ɴt̚] or  [dɪ́t̚ɴt̚] or [dɪ́ʔɪnt̚] depending on the speaker. The best practice is to imitate the native speakers you communicate with and hear in blogs, movies and shows — assuming they speak English reasonably well.