楼主，你好！
你不懂用英文是“You don’t know。”用于口语的话是“You do not get it”
比较口语的说法，you don't get/catch/follow it。书面一点的，you do not understand。
You don't know . You don't understand .都可
You don't know English very well.
You don't use English very well.
You don't speak English very well.
know/use/speak/write/read/... 按语境可以互换。
You don't know that或者you know nothing about that
不明白的话hi我
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