Chinese · Pronunciation Guide
wun, dropping hard from high to low. Lips round to start, a short central vowel, and a front n to finish.
wèn
Nothing changes the fall, though it changes what precedes it: 不问 becomes bú wèn. The relevant contrast to keep in mind is 问 wèn against 闻 wén and 文 wén, which differ only in tone — a falling delivery is the only thing marking this as the verb "to ask".
Listen: 问
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我可以问你一件事吗?
Wǒ kěyǐ wèn nǐ yí jiàn shì ma?
Can I ask you something?
Listen: 我可以问你一件事吗?
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我可以问你一件事吗? opens with wǒ kěyǐ, three consecutive third tones, so wǒ and kě both rise and only yǐ keeps the dip. Then wèn falls, nǐ dips, and yí is 一 converted to a rising tone before the falling jiàn. The sentence has two separate sandhi phenomena in eight syllables, which is fairly typical for Mandarin.
Learners frequently drop the lip-rounding and produce "un", or over-round it into "woon". The lips round briefly and then relax into the central vowel. The nasal is the other issue — wèng with a back nasal is a different syllable, and since the vowel is short and central, the ending is doing a lot of the identifying work.