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plCommon errors
Lexical errors
Learners might use interrogative pronoun 多 with incorrect adjectives.
*这本书有多薄。
这本书有多厚。
*這本書有多薄。
這本書有多厚。
It would be advised to give them enough examples of which adjectives are typically used with interrogative pronoun 多 (高，长/長，大，快，远/遠，重，深，厚, etc.).
Syntactic errors
Learners might add question particle 吗/嗎 at the end of the question.
*你有多高吗？
你有多高？
*你有多高嗎？
你有多高？
Other usages
Phrase with interrogative pronoun 多 and an adjective may also be used as an adverbial or attributive. See example:
你多快能写完你的作业？
你多快能寫完你的作業？
你穿多大的裤子？
你穿多大的褲子？