When you have two independent clauses with a comma joining them, the easiest fix may be to simply separate them. Place end punctuation in the position of the comma and capitalize the first word of the second phrase. Only three end punctuation marks exist, so you can easily determine which one is needed. A period provides closure to a statement. A question mark ends phrases that ask questions or require answers. An exclamation point indicates strong emphasis.
If you want both independent clauses in a single sentence, use a semicolon. A semicolon changes a comma splice into a grammatically correct sentence. Another way to combine the two clauses is to add a conjunction such as "and" or "but" after the comma.
Greg Pensinger, writing for the San José State University Writing Center, explains how to use subordinating conjunctions. These conjunctions are words and phrases such as "before," "now that" and "since." Many subordinating conjunctions exist. They combine independent clauses with relationships such as cause and effect, timing, purpose, conditions, manner or comparison. Combine the two independent clauses by stating the relationship and using a comma where the clauses meet.
Northern Illinois University lists 30 conjunctive adverbs and explains the rule for using them in sentences. When you have two independent clauses, you can use a conjunctive adverb such as "however," "nevertheless," and "therefore" with a semicolon and a comma. End the first clause with the semicolon. Place the conjunctive adverb next and follow it by a comma. The second clause follows the comma. Follow it with end punctuation.
Two additional ways allow you to combine independent clauses and fix a comma splice. One is to use a dash in place of the comma. A dash is composed of two hyphens on a keyboard. If you choose to use the dash, use it sparingly. The second additional comma splice fix is the colon. If the second clause explains or illuminates the first, you can place a colon between them.