participle problem

1.)People dressing conspicuously are stupid.
我想問呢句係咪用左present participle? Yes, very good!你的英文已很好!
Two original simple sentences: the first sentence is:People dress conspicuously .
the second sentence :People are stupid .
跟住就用present participle 連接, = 變成了People dressing conspicuously are stupid.-<係咪就係因為用左present participle的關係所以就變左People dressing conspicuously are stupid? Yes, excellent! 分析正確
分析: 先從 People who dress conspicuously waste money . 入手
present participle is usually used to replace 取代 which/ who/that + verb (present tense or past tense) (= 省去了which/who/ that, 然後將動詞 改為 ing form)

另外想問係咪一定係要用verb to be after People dressing conspicuously ?
No, besides verb to be, you can also use verbs. 可用其他動詞。。
如果我寫,用其他verb得唔得? Yes, of course. 得! 當然可以 用 verb
People dressing conspicuously waste money-<咁我寫呢句又有冇錯?Correct! It is a good example. 冇錯(正確 的)

2.)past participle problem

Vehicles parked illegally may be towed away.

我想問下呢句係咪又係用左past participle? Yes, correct! 對的/ 很正確
分析: the first sentence : vehicles are parked illegally.
the second sentence vehicles may be towed away.

係咪又係用左past participle ,所以變左vehicles parked illegally may be towed away? Right! 理解正確
分析: 先從 Vehicles which are parked illegally may be towed away. 入手
past participle is usually used ro replace 代替 which/ who/that + is/am/are/was/were (省去了which/ who/that 及 is/am/are/was/were)

(1) People dressing conspicuously are stupid.

This is a result of reducing a "defining" relative clause; --
People who dress conspicuously are stupid. (not all the people are stupid!)
During the reduction, "dress" is changed to its present particple, introducing an adjectival phrase to modify the subject "People".

However, the sentence "People dressing conspicuously waste money." doesn't make much sense, because it cannot be derived from a meaningful defining relative clause; ---
People who dress conspicuously waste money. (instead, the two actions "waste" and "dress" have a cause-and-effect relationship)

Therefore, it is more meaningful to say:
People waste money for dressing conspicuously.

(2) Vehicles parked illegally may be towed away.

Same as (1), it is the result of reducing the defining relative clause --- "Vehicles that are parked illegally may be towed away." When the action is in passive voice, we use the verb's past participle in the course of reduction.

2012-06-28 13:05:23 補充：
Alternatively, we can reduce a non-restrictive relative clause in the following manner.
Dressing conspicuously, Peter attracts admiration from his colleagues.

這兩句都是用了分詞子句（participle clause ）
談分詞子句以先，先談談關係子句（relative clause），因為分詞子句，有時看為簡化版的關係子句（如你所例舉的兩句句子）

關係形容詞(relative pronoun)本身就有連詞接與代名詞的功用，如：

That is the boy and he wrote this letter.
我們可以寫成
That is the boy who wrote this letter
也可以寫成
That is the boy writing this letter.

有了這個概念後，相信你也能解決你的問題了
People dressing conspicuously waste money當然是沒有問題

希望幫到你！

2012-06-28 10:50:51 補充：
若你不明白甚麼是關係子句，大可以看英文文化書，關乎關係子句一章

(1)People----Subject
----who (are/were) dressed/dressed---past participle,verbal adj.
----conspicuously-----adv
----are----strong form verb to be=waste (on money)
----stupid,(adj)=most stupid,lack of thought, or good judgement.

(2)vehicles----Subject
-----which are parked----passive voice sentence
-----illegally---adv
-----may be towed---passive voice,
-----away----adv.