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Put simply, past particples may be used as adjectives. What is a past participle?
Past participle
In grammar, the past participle of a verb is a form that is usually the same as the past form and so ends in `-ed'. A number of verbs have irregular past participles, for example `break' - past participle `broken', and `come' - past participle `come'. Past participles are used to form perfect tenses and the passive voice, and **many of them can be used like an adjective** in front of a noun.
Compare: present participle
In grammar, the present participle of a verb is the form which ends in `-ing'. Present participles are used to form continuous tenses, as in `She was wearing a neat blue suit'. They are often nouns, as in `I hate cooking' and `Cooking can be fun'. Many of them can be used like an adjective in front of a noun, as in `their smiling faces'.
參考: Collins Cobuild English Dictionary