關於英文文法的問題?

**** it is about grammar but not logic *******

We will incur some unforeseen expenses once we have relocated.
~ this statement is a prediction or a guess by itself, as it is embedded into the main complex sentence after the word "likely":
Since it is likely that [xxxxx], having at least one credit card would be ideal.
[xxxx] depicts future events which are probably going to happen.

So, "once" is a conditional conjunction in a conditional type-1 sentence.
"once" has the meaning of "as soon as" or "after" and you don't use future tense in the clause with "once" (cf. Cambridge Dictionary).

1. Once we live in the new place, we will incur some unforeseen expenses.
~ for dynamic verbs with progressive action and duration (activity or process), you simply use present tense

2. Once we have relocated, we will incur some unforeseen expenses.
~ for dynamic verbs with momentary action (transitional event or short duration), you have to use present perfect tense to signify the completion of the momentary action.

By the time you come home, I will have finished my homework. 此處come現在式表示主觀認定會發生，以現在式取代未來式，而用will have finished表示在come 之前完成。
此題用will incur表示發生機率50%一半，而後面用have relocated 不用will have relocated只能說主觀認定一定會發生，有強調用意。以現在完成式取代未來完成式。
英語以現在式取代未來式是常用的，表示主觀認定會發生

once, adverb;
once we have relocated=together, at the same time;
in the present, at a time that is stated as present perfect tense;
Hence,:-"once we have relocated".

照邏輯 once we have relocated 的確是 未來式. 但是英語裡面

<副詞子句 習慣上 以現在式來替代 未來式>

所以你的例子 不用: once we will have relocated,
也不用: once we will relocate,


另例:
She will call me when he comes. (O, not he will come)
Kill it if he complains. (O, not will complain)
Open an account with us after you have arrived. (O, not you will have arrived)



可是 你要把她記起來嗎?學語,知道有這回事多念幾次熟了就好. 不必強記.
就算記起來了 在說話的時候 你會找得出來 應用嗎?
你每說一句話,都要翻書20次嗎?

一旦我们搬迁了 once we have relocated
事情你知道是未来，但是once 是假设你已搬迁,所以用 present and past, 而不是用 we had relocated

错了请说，谢谢