be in one's bst interest

"be in one's bst interest" 在書上看到解釋
"為了某人好;為了...的最大利益"

例句給的是
It is in our best interest to buy a house at this time.

我猜中文含意是=>"對我們而言現在是買房的最佳時機"
但此句我一直無法100%明瞭的地方主要在於
我的思考模式一直希望把以下這兩部份合併
 "It is in our best interest"
" buy a house at this time"
 但我怎麼想就是想用個關係代名詞或是for來組合.......但就是合不起來,也不會想到用to , 我怎麼看就是不知道這個to放在這邊要做什麼??

連接詞provided

我在字典裡看到給provided的解釋及例句
conjunction
on the condition or understanding (that); providing:

I'll go provided that the others go, too.

雖然解釋中有"(that)",但我還是想知道有沒有辦法知道為什麼要放那個that?如果沒放可以嗎?我的想法是provided本身就是連接詞,這個that在這個地方的作用是什麼?

[假設語氣]一定要"假設"嗎?

If you ________ thinking we'd profitable again soon, you'd be disappointed.

(a)were
(b)was
(c)are
(d)had been

書上的參考答案是(a),雖然我的因為認為題目是在考假設語氣,所以were會是我的備選答案,但是(c)真的不行嗎?

[假設語氣]使用反於過去的時機

The mayor candidate would rather his opponent __________ about his family.

(1)didn't criticize
(2)wouldn't have criticized

書上的參考答案是(1),原因是反於現在.
但是否可選(2)嗎??,因為我認為也可以用"反於過去".

[假設語氣]could的時態問題

"If I could live longer, I could read more books"

如果不看上下文,能看出是"反於現在"或"反於未來"嗎?