Your ear lies. Play instead.
A big share of the Digital SAT's grammar questions come down to one thing: can you tell where one complete thought ends and the next begins, and punctuate the seam correctly. These are among the most learnable points on the test, and people leave them on the table because "grammar" sounds boring.
Boundary Bridge makes you do only that: play it here. Each round is a sentence with one spot to join and exactly one correct way to join it — comma, semicolon, colon, period, or dash. Pick the right one and the bridge holds. Pick a wrong one and it fails, in a slightly different way depending on what you got wrong.
The reason it works as practice is the "exactly one correct" part. Every item is built so there's a single legal answer, which forces the actual rule — is each side a complete sentence or not — instead of letting you go by which pause "feels right." Your ear is the worst tool for this, and the game takes it away from you.
Run enough rounds and the rules stop being a list you memorized and start being automatic, which is what you want on test day when you've got a minute and a half per question.
One takeaway: SAT punctuation is almost entirely about whether each side of the join is a complete sentence. Learn to check that one thing and most of these questions answer themselves.
Play at forgesat.com/games/boundary-bridge.html. Free, no sign-in. And the free diagnostic will tell you whether punctuation is even where your points are going before you spend time on it — worth the sign-up.
Forge is a beta, built by one developer who still teaches full-time. If an item looks like it has two right answers, tell me — I audit these, but I miss things, and I'll fix it.