樓上說的對,once一般情況下不需要倒裝,不管是作為連詞、副詞還是名詞；
但是,在以否定詞開頭的情況下需要倒裝,如
Not once did he talk to me.
他一次也沒有和我談過.
The old couple have been married for 40 years and never once have they quarreled with each other.
這對老夫妻結婚40年間連一次吵架都沒有.