We often "contract" or shorten words in English. For example, we may say "he's" instead of "he is". Note that we usually insert an apostrophe (') in place of the missing letter or letters in writing. Here are some example sentences:
We do this especially when we speak. We do not contract words so much in writing
These pages show the most common contracted forms.
Contractions are very common in spoken English. They are not so common in written English. We may use contractions in a friendly letter, for example, but they are not usually correct in more formal texts such as business letters or essays. If you have to write an essay in an exam, do not use contractions. The only exception to this would be when you quote somebody within your essay, for example spoken dialogue.