1. The tones — first and always
妈 mā, 麻 má, 马 mǎ, 骂 mà are four different words. A child who learns a word without its tone has learnt it wrong, and it is far harder to correct later than to get right now. Four- to six-year-olds copy pitch better than adults ever will — this is the one thing that is genuinely easier young.