However Is an Adverb, Not a Conjunction
Words like however, therefore, moreover, consequently, nonetheless, furthermore, and thus are conjunctive adverbs. They carry meaning about how two ideas relate (contrast, cause, addition), but grammatically they are adverbs, not joining words. That is the whole trap.
Compare them to the real conjunctions, the FANBOYS: and, but, or, for, nor, so, yet. A FANBOYS word is strong enough to join two sentences with just a comma in front: "The data looked strong, but the sample was tiny." Legal. Swap in a conjunctive adverb and keep the same comma, and it collapses: "The data looked strong, however the sample was tiny." Illegal, because however cannot do a conjunction's job.
Students miss this because however means almost the same thing as but, so the ear treats them as interchangeable. They are not interchangeable in punctuation. Meaning and mechanics are two different questions, and the SAT is testing the mechanics.
The Fix: Semicolon, However, Comma
When a conjunctive adverb links two independent clauses, the pattern is fixed:
independent clause ; however , independent clause
A semicolon (or a period) before the transition word, a comma after it. Both of these are correct:
The prototype passed every stress test; however, it failed in the rain. The prototype passed every stress test. However, it failed in the rain.
The semicolon and the period are doing the identical job here: separating two complete sentences. The comma after however is a light pause that sets the transition word off from the clause it introduces.
The tool that proves it is the Period Swap. Take the mark that sits before the transition word and mentally replace it with a period, then read both halves. "The prototype passed every stress test." Complete. "However, it failed in the rain." Complete. Two complete sentences means the boundary between them must be a period or a semicolon, never a lone comma. The Period Swap is the same move that decides semicolons and comma splices in general, so it is worth owning cold. The semicolon rules guide drills that mark, and the whole family of boundary checks lives in the four grammar rules guide.
When However Sits Mid-Sentence: Interrupter Commas
Here is a second pattern, and where a lot of points leak. Sometimes the transition word is not standing between two clauses at all. It has been dropped into the middle of a single clause as an aside:
The prototype, however, failed in the rain.
Now however is a parenthetical interrupter. It is extra information tucked inside one sentence, so it takes a comma on both sides, exactly like any nonessential element. Prove it with the Delete Test: lift out "however" and read what remains. "The prototype failed in the rain." Still a complete, true sentence. Whatever you can delete cleanly gets fenced off with commas. That is the same logic behind all nonessential punctuation.
So the position of the word decides the punctuation:
- Between two sentences (joining them): semicolon or period before, comma after.
- Inside one sentence (interrupting it): a comma on each side.
Do not let the mid-sentence version fool you into thinking a single comma before however is ever enough to join two clauses. It is not. The two-comma interrupter only works because there is one clause, not two.
The Comma-Splice Trap It Creates
This rule mostly appears on the test as a disguised comma splice: two complete sentences joined by nothing but a comma, which is always wrong. The SAT builds one and hides it behind a transition word, betting your ear hears however as "but" and waves the comma through.
Wrong: "She trained for months, therefore she expected to place." Right: "She trained for months; therefore, she expected to place."
Both sides of that sentence are independent ("She trained for months" / "she expected to place"), so the comma is doing a period's job, which a comma is not allowed to do. The presence of therefore does not rescue it, because therefore is an adverb, not a conjunction. This is a common source of wasted points: the sentence sounds connected, so students never run the mechanical check that would have caught it.
One clean line to memorize: a comma before a conjunctive adverb that is joining two clauses is always wrong. Every time you see however, therefore, or moreover with a comma in front and a full sentence on each side, that is the answer to fix.
Worked Examples
Read each one, decide, then check.
1. "The museum closed early, moreover the parking lot was full." Two complete sentences ("The museum closed early" / "the parking lot was full") welded with a comma. Moreover is an adverb. Fix the boundary: "The museum closed early; moreover, the parking lot was full."
2. "The route was longer, however, the traffic was lighter, so we made better time." The tempting read treats "however" as an interrupter, but "The route was longer" and "the traffic was lighter" are both full sentences, so however is joining them, not interrupting one. It needs a semicolon before it: "The route was longer; however, the traffic was lighter, so we made better time."
3. "The findings, therefore, changed the entire proposal." Delete Test: remove "therefore" and you still have "The findings changed the entire proposal," one complete sentence. Here therefore really is a mid-sentence interrupter, so the two commas are correct. Nothing to fix.
Notice that examples 2 and 3 use the same word in the same middle-ish position, and the punctuation is different, because the number of clauses is different. That is the entire skill.
Self-Test: Run the Period Swap on Yourself
In ten minutes with your last practice section, find every however, therefore, moreover, consequently, nonetheless, and thus in the questions you missed. For each, ask two questions in order: (1) Is there a complete sentence on both sides? If yes, the mark before the transition word must be a semicolon or period, comma after. (2) If there is only one clause and the word is an aside, does the Delete Test pass? If yes, two commas.
If your misses cluster on step 1, your leak is the comma splice hiding behind a transition word, and the comma splice guide is your fastest fix. Transition words also carry a separate skill, choosing the right one for the logic, which is a different question type covered in the transitions guide. Punctuation decides where the mark goes; the transition question decides which word belongs.
Practice it now: the free simplesat grammar drill runs exactly these boundary checks with per-trap feedback, so you can see whether the transition-word splice is one of yours. No signup, works as a guest.