In 1998, Jakob Nielsen published "Microcontent Guidelines," a short essay on "How to Write Headlines, Page Titles, and Subject Lines" ("pearls of clarity...40-60 characters to explain your macrocontent").
I absolutely took the essay to heart and I still refer to its precepts when writing subject lines, headlines, etc.
Anthony Diké's "How To Write Great Microcopy" is an interesting successor to Nielsen's 22-year-old essay.
"Microcopy" isn't exactly the same thing as "microcontent" - much of Dike's work focuses on persuasive marketing copy. But he's got a lot to say about other kinds of short text, including UI elements, error messages, etc.
A selection of my favorites:
* (Almost) always use the active voice
It’s stronger and easier to understand than the passive voice.
Use it when you need to signal who or what caused an action.
* Use the passive voice (sometimes)
It has its place.
Use it when the action is more important than what caused (subject) the action.
* Keep it scannable
Reading is work. Every word takes energy. Users like to save energy by skimming.
* Avoid destructive feedback
It’s unhelpful and depressing.
He's also collected them all into a single Twitter thread:
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