In grammar, the use of the colon (:) is common to introduce a list of items, an explanation, and even a quotation. However, in all of these cases, the colon can only be used if the sentence or clause before the colon is complete or expresses a complete idea and contains at least a subject and a verb. This is a rule because the colon as any other punctuation mark represents a break in writing and by placing it in an incomplete or dependent clause, essential elements of the sentence would be broken or divided which is not recommended or correct. Therefore, to use a colon to introduce a quotation you must have a complete sentence before the quotation.
