adverb 應放在句子的那裡 ???

He crosses the street slowly.
Slowly he crosses the street.
He slowly crosses the street.

以上三句都是正確的，但意思其實有少許分別。

第一句主要是說他過馬路，不過多寫出是慢慢地過。
第二和第三句是強調他慢慢走。

重點有少許分別。

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All are correct. The only differences are the emphasis and writing style.

Quietly and quickly, the thief stole all valubles from the house. (You put the emphasis on 'quietly and slowly')

He slowly walked down the hill. (The emphasis is on the speed of the walking)

He walked down the street slowly (The emphasis is on 'walk' and you describe it with 'slowly')

It doesn't matter. For example:

He crosses the street slowly.
Slowly he crosses the street.
He slowly crosses the street.

They are the same.