If you run out of time, unanswered questions are marked wrong — there's no partial credit and no guessing penalty, so always fill in an answer for every question.
Any question you don't answer is scored as wrong. There is no partial credit. There is also no penalty for guessing, which is why leaving anything blank is strictly worse than picking an answer at random.
If you're seconds from the end of a module, pick a letter and fill it in for every remaining question. That alone typically adds a few raw points compared to leaving them blank.
Drill free SAT questions by skill, take timed modules, and project your 1600-scale score.