Only when accidents happen do people regret not having been more careful. (只有當意外發生時, 人們才會後悔之前沒有更小心), 請問為什麼此句要用到 having been, 為何要寫?

I regret being unable to help you.(用簡單式動名詞being,表示與主要動詞無時差:狀況一直是如此,我幫不了你)
He regretted not having come oftener to her.(用完成式動名詞having come,表示與主要動詞有時差,該狀況是一個更早的狀況,他現在雖然常來了,但以前沒有常來)
He denies having been there.（用完成式having been，因為時間點比主要動詞早）
He was accused of having deserted his ship.（用完成式having deserted，比主要動詞早）
The safe showed no signs of having been touched.（用完成式被動態，比主要動詞早）

如果是表示action的動名詞，有時可從字義上判斷與主要動詞有無時差，不致造成誤會：
He suffered without crying. (動名詞與主要動詞同時)
I am sure of his coming. (比主要動詞晚)
I regret telling him the truth. (按邏輯，telling比主要動詞regret早；所以雖未用完成式，但不致造成誤會)

Only when accidents happen do people regret not having been more careful.
有時差，所以必須用完成式動名詞having been當受詞來分出事件的先後。（人們是現在對「過去的」不小心感到後悔）。be是state verb，動名詞用簡單式being時，是指目前的狀況；用完成式having been是指從前的狀況。

當我們要表達與過去事實相反時，會採用完成式。
If only I had been more careful that wouldn't have happened.

文法基本教義派或講求規矩的英國紳士多數會採用 regret having doing 來表示過去，特別是要強調 "早知如此，何必當初" 的懊悔，因為這可連結到假設語法。
I regret not having worked harder, because now I am poor, and my children did not receive an education.

現代美語，許多人會直接採用動名詞。但你必須了解完成式是有它的特殊用意。尤其是牽涉到 "假設語法"

先從＂正面＂的角度來看看類似的說法要怎用：

１．人們（都很）小心
people were careful.

２．人們（都更加）小心
people were more careful.

３．人們已經小心
people have been careful.

4．人們應該要（但是沒有）小心
people should have been careful.

5．人們應該要（但是沒有）更加小心
people should have been more careful.

6．改詞性為＜名詞子句＞已更加小心
having been more careful.

７．＜名詞子句＞沒有已更加小心
not having been more careful.

８．＜＜動詞＞＋＜名詞子句＞沒有已更加小心
regret not having been more careful.

＜完成式＞常常被用在＂應該卻沒有＂的事實描述．

有沒有做一件事情要用完成式have+V-pp
然後regret後面要+V-ing，所以用having