1. A relative clause is an adjective clause.
2. A reduced clause is a phrase.
3. When you are not sure if you can reduce the clause or not, then don't.
4. Usually only one-word participles can be put in front of the noun:
a. The wolves howling tonight must be hungry... The howling wolves tonight must be hungry.
Look at these examples of relative (adjective) clauses:
1a. People who buy lottery tickets are often found at bingo.
1b. People buying lottery tickets are often found at bingo.
2a. The students who were waiting for their funding from the government were disappointed.
2b. The students waiting for their funding from the government were disappointed.
3a. Those students who want to go to Big White for snowboarding need to pay soon.
3b. Those students wanting to go to Big White for snowboarding need to pay soon.
In the sentences above, the relative pronoun (who, which, that) can be omitted along with the verb to be and replaced by the present participle (-ing). Notice that the simple present verb tense changes to the present participle.
In 1b, the verb in the clause expresses a habitual or continuous action, something that happens on a regular basis. Other examples:
a. Kids playing in the streets may get run over. = Kids who play
b. ESL students attending summer session must register by Friday. = students who will attend
However, hobbies and repeated actions cannot take this construction:
c. People who play golf are always prepared for inclement weather.
(compare: The people playing golf today are getting wet from all the rain.)
d. Students who dont practice their English dont improve very quickly.
(compare: Students not practicing their English during class time today will be punished.)
e. The bus which leaves at 6:03 was late today, so I caught the 6:08 one for downtown.
(compare: *The bus leaving at 6:03 was late today.)
In 2b, the verb is in the continuous or progressive tense and can be replaced by the present participle:
f. I am waiting for the student who is writing his final exam early. = for the student writing
g. Give high marks to the students who are speaking English in class today. = the students speaking
In 3b, verbs like wish, desire, want, and hope (not like) can be used in this way.
h. Those students desiring a second grammar course please sign up at my office.
i. All the students wishing for a better grade are here studying hard.
Helpful hint: When you are not sure, put in the who, which, or that.
Note: These examples are used for defining or definite relative clauses.
Grammar in Context by Hugh Gethin ISBN 0 00 370025 9
A Practical English Grammar by Thompson & Martinet ISBN 0 19 431336 0