A semicolon should be used to join two complete sentences. However, it can't be any two sentences. The sentences must be closely related to warrant using a semicolon. Sentences that are not closely related should be separated by a period. Example: The dog ate my homework; I'm going to get an F.
Use a semicolon before introductory words like therefore, however, in addition, instead, moreover, subsequently, additionally and consequently when they connect two complete sentences. These words are also called conjunctive adverbs. Example: the dog ate my homework; therefore, I'm going to get an F.
Some sentences contain long, punctuated phrases or lists with extra commas to set off units in a series. These sentences need semicolons to make the sentence easier to read. Example: The dog ate my homework, which was for English; my term paper, which was for history; and my test, which was for science.
Long and complicated sentences often contain multiple phrases with multiple commas. Like punctuated lists, these types of sentences require semicolons to make them easier to read. Example: the dog ate my homework, test and term paper; but my teacher, for some reason, decided to give me an A.
Semicolons should be used between two sentences connected by a coordinating conjunction if commas are present in the first sentence. Example: When I finish retyping the assignments that my dog ate, I can take you to the store; and then we can stop for ice cream.