感謝He left the engine which was running.解惑,另She kept her boyfriend waiting for over an hour waiting 是形容詞片語,展為who was waiting for over an hour?

1. He left the engine which was running.
===> "running" is the present participle component in the past continuous tense

"leave"如解 "離開" (及物動詞), 一般用于地方、人 或 工作(辭去) 作為受詞.
examples-
to leave home; to leave London; to leave his wife; to leave her teaching profession;

He left the engine... (他離開了發動機/引擎)  是解不通的.
He left the engine (locomotive) which was running. (他離開了 行走中的 機車/火車頭)  (OK)

He left the engine running.  他讓引擎(繼續)運轉著.
(= He allowed the engine to remain running.)
===> "running" is a present participle and it is adjectival (形容詞性質的)
Format: "leave" + direct object + V-ing (present participle)

2. She kept her boyfriend waiting for over an hour. 她讓她男友一直等超過一小時.
(= She made her boyfriend remain waiting for over an hour.)
===> "waiting" is a present participle (adjectival) and it represents a continuous action of the direct object of "keep"  (代表受詞繼續進行中的動作)
Format: "keep" + direct object + V-ing (present participle)

展為who was waiting for over an hour?No, the participle phrase (waiting for ...) is not a reduction of relative clause.

佢留番佢的男朋友 past continuous tense 等待了一个多小时-----相對的詞叫-喺非定義條款, she 之際 was absent .

1. He left the engine which was running.
=他離開了那個正在運轉中的引擎。
which was running是修飾名詞engine的「修飾語」，是「形容詞子句」。
He left the engine running.=他走時，沒關掉引擎。
running是「受詞補語」。是形容詞性質的「現在分詞」。
He left the engine while it was running.這樣寫時，while it was running是「副詞子句」修飾前面的「述部」left the engine。引擎還在運轉時，他就走了。

2. keep xxx ~ing
I'm sorry I have kept you waiting so long.
waiting是「受詞補語」。
使...保持...的狀態
She kept her boyfriend waiting for over an hour.
是to keep someone doing something for how long的關係。
她讓男友保持等待狀態達一小時以上。