In "他常在电脑上练习用汉语写东西", the location '电脑上' is placed before the verb '练习'
他常 (he often)
在电脑上(on computer) -- location
练习 practice -- verb
用汉语写东西 -- relative clause
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In "他常练习在电脑上用汉语写东西" , the location '电脑上' is placed before the verb phrase '用汉语写东西'
他常(he often)
练习 (verb)
在电脑上用汉语写东西 -- relative clause
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在电脑上 location
用汉语写东西 -- verb phrase (the verb is 写)
Both sentences mean "He often practices writing in Chinese on a computer". The difference is in the first sentence the location '在电脑上' is a relative phrase for the main verb '练习' (在电脑上练习) ; and in the second sentence, the location '在电脑上' is a relative phrase for the verb in the relative clause '用汉语写东西' (在电脑上用汉语写东西)