Chinese · Pronunciation Guide
ü, low and sagging — the rounded front vowel, not a plain "oo". Set your tongue for "ee", then purse your lips without letting the tongue move.
yǔ
The neighbour decides whether this dips or rises, and since rain is usually the object of 下, the syllable often ends a clause and gets the full dipping contour with its final lift. The spelling point matters as much as the tone: after y, the letter u always represents ü, so yu is never "yoo".
Listen: 雨
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外面在下雨。
Wàimiàn zài xià yǔ.
It's raining outside.
Listen: 外面在下雨。
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外面在下雨。 ends on yǔ, so the dip completes with its lift. Before it, wài, miàn, zài and xià all fall — four consecutive descents, each needing its own reset to the top. The sentence therefore drops repeatedly and then finally sinks to the floor, which is about as downward-moving as Mandarin gets.
"Yoo" is the near-universal rendering and it uses a vowel that is simply not in this syllable. The tongue must stay high and front while the lips round — if it slides back, you have made a plain u. And the four-fall run in the example sentence gets levelled into one long slide by almost every beginner.