You never mean: fall between the cracks
You always mean: fall through the cracks
Why: Logically speaking, you can’t fall between an opening; you fall into or through it. As such, don’t let logic fall through the cracks when you use this idiom.
You might say: mute
You might mean: moot
Why: Moot‘s most common meaning is “deprived of practical significance.” Mute means silent. So while they might want to be mute about a moot point, careful speakers will be vocal about the difference.
You almost never mean: just desserts
You almost always mean: just deserts
Why: When you want someone to get what he deserves, you hope he reaps his just deserts. But on her birthday, a particularly well-behaved three-year-old might be allowed in just desserts.
You never mean: modern-day
You always mean: modern
Why: Quite simply, modern covers it. Modern-day is a redundant term. Modern speakers will sound smarter by using the superior word.
You might write: so-called “good grammar”
You mean to write: so-called good grammar
Why: So-called introduces a term as falsely, improperly, or commonly referred to as something. So by default, it covers the need for quotation marks (or a speaker’s air quotes).