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Search for this and you get two different systems tangled together: the Roman-numeral outline instructors assign, and APA's actual five-level heading rules for the finished paper. Here is each level, exactly as APA defines it, plus how the two systems map onto each other.
Six grammar errors show up in research papers more than anywhere else, because academic sentences are built to hold more qualification than an everyday paragraph can carry. Here is the mechanism behind each one, a wrong-and-fixed example, and what a free grammar checker can and cannot catch on its own.
A results section reports a number. A discussion section is supposed to argue for what it means. AI flattens the difference between the two, and the giveaway is hedging stacked so thick the sentence stops claiming anything at all.
Oxford University Press spells organize with a z in British English, on etymological grounds, which is why the convention is called Oxford spelling rather than an Americanism. A closed set of verbs, advertise and surprise among them, never takes -ize under any style, for a completely different reason worth knowing before a proofread turns a correct spelling into an error.
APA 7's et al. rule is simpler than the confusion around it: three or more authors, et al. from the first citation, no exceptions except two-author sources. Here is what changed from APA 6, the edge cases that trip people up, and where the full stop actually goes.
A methods section is formulaic by design, which is exactly why an AI-drafted one reads as machine-written. Here is what to leave alone, what is actually yours to vary, and how to tell the two apart section by section.