What is an idiom?

If you’re going to converse with English speakers, you’ll need to master our most commonly used idioms. What’s an idiom, you ask? Google.com’s handy dictionary offers the simplest explanation:
Idiom (noun): a group of words established by usage as having a meaning not deducible from those of the individual words (e.g., rain cats and dogs, see the light).
The thing about idioms is that they’re often scrambled, mixed, and lost in translation! Here are some of the most common mix-ups. Don’t miss the grammar mistakes you could be making.
Bat vs. back

Right: Right off the bat
Wrong: Right off the back
What the idiom means: From the very beginning…
Here are 70 more words and phrases you’re probably getting wrong.
Ear vs. year

Right: Play it by ear
Wrong: Play it by year
What the idiom means: Don’t make solid plans; just see what unfolds.