A comma splice occurs when two independent clauses are joined by only a comma, without a coordinating conjunction (The rain stopped, we went outside). A fused sentence (run-on) joins two independent clauses with no punctuation or conjunction at all (The rain stopped we went outside). Both errors signal that the writer has not recognized the boundary between two complete thoughts. Fixes include adding a coordinating conjunction after the comma, replacing the comma with a semicolon, separating into two sentences, or subordinating one clause.
Identify the independent clauses first — each must have its own subject and verb and be able to stand alone. Once the clause boundary is found, evaluate which repair strategy best fits the relationship between the two ideas: coordination, subordination, or separation.
You already know that a compound sentence joins two independent clauses, and that each independent clause can stand alone as a complete sentence. The question that comma splices and run-ons raise is: what happens when a writer tries to join those clauses without the right connective tissue? Both errors stem from the same underlying confusion — failing to recognize that each independent clause is a full thought, not just a fragment that can be glued to its neighbor.
A comma splice inserts a comma where the junction demands more. "The rain stopped, we went outside" uses a comma, which does signal a pause, but a comma alone is not strong enough to bridge two independent clauses. Think of punctuation marks as having different "bonding strengths": a period fully separates two sentences; a semicolon joins two closely related ones without a conjunction; a comma with a coordinating conjunction (FANBOYS: for, and, nor, but, or, yet, so) forms a compound sentence. A plain comma between two independent clauses falls below the minimum required bond strength. The sentence looks connected but grammatically it is two sentences pressed together by the wrong fastener.
A fused sentence (or run-on) is the more severe version: no punctuation at all, and no conjunction either. "The rain stopped we went outside" simply collides the two clauses. Every fused sentence is also a run-on, but not every run-on is fused — comma splices count as run-ons too. The practical fix for either error is to identify the clause boundary first, then choose a repair strategy that reflects the logical relationship between the two ideas: a period if the ideas are separate; a semicolon if they are closely linked; a coordinating conjunction with a comma to make the relationship explicit (contrast, addition, causation); or subordination to demote one clause ("After the rain stopped, we went outside").
The conjunctive adverb trap is worth understanding carefully. Words like "however," "therefore," "consequently," and "moreover" look like they could function as conjunctions, but grammatically they are adverbs, not connectors. They describe the relationship between clauses but cannot join them on their own. So "The rain stopped, however we went outside" is still a comma splice — "however" does not rescue it. The correct forms are either "The rain stopped; however, we went outside" (semicolon before, comma after) or two separate sentences. Knowing this distinction lets you fix a whole class of errors that appear in academic and professional writing, where these transitional adverbs are heavily used.