A contraction is when two words are joined and one or more letters are removed and replaced with an apostrophe. There are two types of contractions in English and both types are formed only with the verbs BE, DO, HAVE or a modal verb. These contractions are very common in speech and writing.
1) Contraction of subject pronoun + verb.
Note: the verb DO cannot contract with subject pronouns.
2) Contraction of verb + negative adverb not.