Buttons
A button should say what it does, not give assent. "Send request" beats "OK", because users often read the button before the question.
Corollary, in a destructive confirmation: the cancel button must not say "Cancel". In "Cancel your subscription? — Cancel / Confirm", nobody knows what "Cancel" cancels. Write "Keep my subscription" and "End subscription".
Error messages
Three elements, in order: what happened, why, and what to do now. "Error 402" contains none of them.
And never blame. "Invalid address" accuses; "The address is missing an @" informs. The second measurably reduces form abandonment.
Empty states
An empty screen on first use is a decisive moment, and almost always neglected. It should not say "No items" but explain what will appear there, and offer the first action.
Placeholder text inside fields never replaces a label. It disappears on typing: a user rereading their entry no longer knows what was asked, and screen readers do not reliably announce it.
Tone
Humour in an interface ages badly and intrudes at the wrong moment: nobody wants a joke when a payment fails. Warm neutrality lasts better than displayed personality.
How to test
Read the text aloud. If you would not say it to someone in front of you, it is wrong. It is the fastest and most reliable test in the field.
our piece on UX design sets microcopy among the activities of the profession.