Why two styles exist at all
APA belongs to the American Psychological Association and grew up in the sciences, where the question that matters about a source is how recent it is. That is why the year sits directly after the author's name in every citation. MLA belongs to the Modern Language Association and grew up in literature, where a book from 1847 can matter more than one from last week, so the year retreats to the end and the page number takes center stage.
Which one you use is decided by your course, not by preference. Psychology, education, nursing, business and the social sciences run on APA. English, literature, languages, and most humanities run on MLA. If the syllabus does not say, ask, because guessing wrong means reformatting every citation in the paper.
The same book, cited both ways
Look closely and four differences are visible in one line: the first name (initial vs written out), the year's position (up front vs at the end), the title capitalization (sentence case vs title case), and the punctuation carrying it all.
Every difference that matters
In-text citations, where everyone slips
This is the difference professors spot from across the room. APA is author-comma-year; MLA is author-space-page with nothing else:
Two things to burn in: APA requires the page number only for direct quotes, while MLA wants the page for paraphrases too whenever the source has pages. And MLA never writes p. before the number. If you catch yourself typing (Brown, 87) or (Brown 2021), you have crossed the streams.
The same website, cited both ways
Notice what each style italicizes on a website. APA italicizes the page title and leaves the site name plain. MLA does the exact opposite: the page title goes in quotation marks and the site name gets the italics, because in MLA the site is the container. This single reversal causes more mixed-up citations than anything else.
Paper formatting differences
How to keep them straight
One memory hook covers most of it. APA is the science style, and science cares about when: so the date rides up front, first names shrink to initials the way lab papers do, and only the first word of a title gets a capital, like a sentence reporting a finding. MLA is the literature style, and literature cares about where in the text: so the page number leads, authors keep their full names the way they appear on book covers, and titles keep their Title Case dignity.
And when a class switches on you mid-semester, do not reformat by hand. Paste the source into the generator, click the other style, and read the output against the rules above so you understand what changed.