Final answer:
Periods are used at the end of declarative sentences, imperative sentences, indirect questions, and polite requests. They go inside quotation marks unless a source is cited, in which case they come after the citation.
Step-by-step explanation:
Periods are used to end a variety of sentences in the English language. They typically signify the end of declarative sentences, making statements of fact or opinion, and imperative sentences that issue commands or requests. Additionally, periods are employed to conclude indirect questions, which are statements that report a question without directly asking it, and polite requests that are framed as statements. Periods should be placed inside quotation marks when no source is cited, and if citing a source, the citation comes before the final period of the sentence. Conversely, exclamation points end sentences that are emphatic or convey strong emotion.