some english problem

(1)main verb 係were
(2)但唔當stay係past participle

"the rooms we stayed in" 係subject;
而"we stayed in" 係補充 the rooms嘅extra 資料
句野最簡化係咁:
The rooms were without air-conditioning.
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(簡化少少句句子):
The rooms we stayed in were nice.
The rooms we were staying in were nice.
The rooms we are staying in are nice.
The rooms we have been staying in are nice.

以上嘅main verb 都係verb-to-be (were/ are).
補充緊the rooms ge, 唔係past participle;
其實全部都食左個"which/that": (assume 你識relative clause)
i.e. The rooms (which) we stayed in were nice. (stayed : past tense)
i.e. The rooms (which) we were staying in were nice. ( were staying: past continuous)
i.e. The rooms (which) we are staying in are nice.(are staying: present continuous)
i.e. The rooms (which) we have been staying in are nice. (have been staying: present perfect continuous)
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在以下情況, 補充個主角ge, 先係past particple (assume你識passive voice被動式):
The butterfly caught by the child is purple. ( The butterfly were caught by the child. : passive voice, verb 變past participle)
The words written on the blackboard were too small. (The words were written by a person. : verb 變past participle )

2012-08-03 17:45:37 補充：
麻煩曬各位GE修改!

The rooms= subj
we stayed in=defining re.cl.
were=v.

In addition, the rooms (where) we stayed in were without air-conditioning.
In addition, the rooms (that) we stayed in were without air-conditioning.
In addition, the rooms in which we stayed were without air-conditioning.

where we stayed in ~ defining relative clause

題目的句子，是將兩句句子合併成為一句。
We stayed in some rooms. (stayed 是 verb -> past tense)
The rooms were without air-conditioning. (were 是 verb -> past tense)

將兩句句子合併時，要決定那句是主句。
你的例子中，The rooms were without air-conditioning. 是主句，
而We stayed in some rooms. 作子句 clause.

2012-08-02 21:03:40 補充：
In addition , the rooms (that) we stayed in were without air-conditioning.

句子中的 (that) 隱藏了。