Here’s a rather stark example of how learning standardized Mandarin (pǔtōnghuà = 普通话) is a non-trivial exercise in learning new phonemes for speakers of other Chinese languages. This question is from the Peking University language forums, so you might guess the person asking is a relatively* sophisticated language learner:

普通话的“你”的声母到底是舌尖的n，还是舌面的gn？我以前一直以为是舌面的gni哦，结果听到一位同事读边音li，他竟然说他也能分清楚li和ni。还说北京话的”你”就是“ni”而不是“gni”。一般情况下，我分不清n和l，但能分清gn和l，所以我一直以为普通话的“你”是gni，因为我一听鼻音就认为声母肯定是gn

In pǔtōnghuà is the initial consonant of “nǐ” actually an apical (tongue-tip) sound ‘n’ or a laminal (tongue-blade) sound ‘gn’**? I used to always think it was gni, but then I heard a coworker read it as a lateral, “li”. Surprisingly, he said he could distinguish “li” and “ni”. He also said Beijing dialect pronounces it “ni” and not “gni”. Usually, I can’t distinguish “n” and “l”, so I always thought that 你/nǐ in pǔtōnghuà was “gni” because as soon as I heard the nasal I thought it was “gn.”

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*Relatively sophisticated, but it’s hard to be sure how sophisticated. For example, I’m correcting the use of “gn” to “ng” to indicate ŋ. But heck, maybe I’m not sophisticated enough myself and there’s something about “gn” that I should know…

**Big correction: I misunderstood the “gn” notation and got corrected by Chrix in the comments below. The whole thing makes a lot more sense when you realize the writer’s “gn” stands for ɲ (IPA) like the ñ in mañana (Spanish) or the gn in gnocchi (Italian). It makes me question my own ears: is it possible that northerners say ɲihao? Does anyone have a good recording of ɲihao?

Say you come to China speaking no Mandarin. You settle into some place where the Mandarin spoken is not exactly pǔtōnghuà (普通话 =  standard Mandarin as it’s “supposed” to be spoken). Might be out in the countryside. Might be in a city where the dominant language is, say, Wu rather than Mandarin. Might even be in Beijing if you get deep enough into the hutong.

Inevitably you come across someone who tells you you’re saying things wrong: maybe they tell you “10” is not sí the way you and the locals pronounce it, but is supposed to be shí; maybe they say “blood” is xiě or xuè rather than xuě as everyone you know says it.

But you are linguistically enlightened. Continue…