大相径庭

dà xiāng jìng tíng

poles apart; completely different

Character by character

大dà
greatly

相xiāng
mutually

径jìng
path

庭tíng
courtyard

现实和想象往往大相径庭。

Reality is often worlds apart from what we imagine.

Tone: neutral.

Like a narrow path versus a courtyard. 相 is xiāng. Often A与B大相径庭.

Learn it in context

This idiom and thousands more are in Learn Chinese with Bao, with audio, pinyin and drills that make them stick.