We will have to stand at the b

摁! 聽起來很順，所以我想是對的。。

一般搞不太清楚過去/未來式，你可以劃一條timeline把發生的事的時間點整理出來，應該就可以清楚看見兩者用法的差別。

未來完成式: volunteers will have taken...
是你現在推斷自願者(Volunteers) 應該已經佔滿所有位置了

未來式 應該是指: We will have to stand...這個部分吧!
用在這裡是對的。因為如果你的推斷是正確的話，你們才會站在觀眾席(auditorium) 的後方。這是未來式，因為你只是推斷/猜測，位置應該是被佔滿了。

如果另一種說法也可以，給你參考。
這是在已經確定的情況下。。
The volunteers have taken all the seats by now, so we now have to stand at the back of the auditorium.

...will have taken all the seats by now
這句是完全正確的

是也可以說
...should have taken all the seats by now

但兩者皆可

給003大大

是 should/will have taken [ALL] of the seats by now
你的ALL不見了

完全正確

because從屬連接詞帶領從屬子句

the volunteers will have taken all of the seats by now.做為主要子句

We will have to stand at the back of the auditorium 的原因

will have + pp 〈早就已經........


[ because the volunteers...by now] 那一句是從屬子句, 不是主要子句哦

We will have to stand at the back of the auditorium because the volunteers will have taken all of the seats by now.
>> 這句有個地方不順就是: will have taken of the seats by now
似乎應該改為: should have taken of the seats by now
因為 by now 是說到現在已經怎樣, 所以我認為座位現在應該已被佔有, 所以應該用較肯定的語氣 should have taken 而非未來完成式(will have taken)
(供您參考)

完全正確

because從屬連接詞帶領從屬子句

the volunteers will have taken all of the seats by now.做為主要子句

We will have to stand at the back of the auditorium 的原因

will have + pp 〈早就已經........