These things are all incredibly common and worth memorizing. Many thanks to @emelie_burnette (our amazing Copy Chief) for teaching me all of this wisdom!
  1. When writing about the 1950s, for example, don't put the apostrophe in the wrong place (50's = wrong // '50s = right).
    The apostrophe goes BEFORE the 5 because it's standing in for the missing numbers (1 and 9) of 1950s. If you're starting a sentence, you should spell it out: Fifties. Also: if you use "1950s," you don't need an apostrophe btwn the 0 and the s.
  2. Everyday vs every day
    "Everyday" is an adjective that modifies something like an outfit or a routine ("my everyday beauty routine" for example), while "every day" refers to a period of time ("I drink champagne every day").
  3. Peek/peak/pique
    How to remember: PEEK is what you do with your EYES, and just like the word "eyes," peek has two Es. PEAK refers to mountain tops, amongst other things, and you can remember because the A in PEAK looks a wee bit like a mountain. PIQUE (the verb) is the one that has to do with curiosity/interest, usually. (Pique as a verb can also mean irritated, and pique as a noun means a feeling of being slighted and annoyed by said slight, basically.)
  4. It's vs. its
    I get why people flub it, "it's" looks possessive, but it's actually a contraction. "It's" = "it is"
  5. That vs. Who
    When you're referring to a person, it's generally better to use "who" rather than the dehumanizing "that." There are exceptions to the rule, but use "who" when talking about a person and "that" when talking about an item/object.
  6. Then vs. Than
    Then = Time "We went to Trois Familia for potato tacos and then headed to Churro Borough for ice cream." Than = Comparison "The pancakes at Jon & Vinny's are better than the ones at Ysabel."
  7. Hyphens: figure it out.
    Hyphens are super tricky, so when in doubt I usually run it through and copy whatever they did. Generally used to link two attributive words that modify a noun, like: long-term relationship.