很高興為您解答如下：
Only then _______ what he really meant when he said “ You left something here.”
A.did I understand B.I understood
C.understood I D.did understand I
答案：【A】
這里考察的是Only位于句首的倒裝結(jié)構(gòu).
1 Only位于句首修飾狀語(yǔ)要倒裝
Only in this way can you learn english well
2 在句首的only如果后面跟了副詞、介詞短語(yǔ)或從句等狀語(yǔ),則主句用倒裝結(jié)構(gòu).
Only when the war was over in 1918 was he able to get back to work．
本句中為only后接了時(shí)間副詞then,所以主句用倒裝,謂語(yǔ)動(dòng)詞借助于助動(dòng)詞do實(shí)現(xiàn)倒裝 根據(jù)一般過(guò)去時(shí)態(tài)用did.
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