Non-Finite Verbs:
In linguistics, a non-finite verb (or a verbal) is a verb form that is not limited by a subject; and more generally, it is not fully inflected by categories that are marked inflectionally in language, such as tense, aspect, mood, number, gender, and person. As a result, a non-finite verb cannot generally serve as the main verb in an independent clause; rather, it heads a non-finite clause.
By some accounts, a non-finite verb acts simultaneously as a verb and as another part of speech; it can take adverbs and certain kinds of verb arguments, producing a verbal phrase (i.e., non-finite clause), and this phrase then plays a different role — usually noun, adjective, or adverb — in a greater clause. This is the reason for the term verbal; non-finite verbs have traditionally been classified as verbal nouns, verbal adjectives, or verbal adverbs.
English has three kinds of verbals: participles, which function as adjectives; gerunds, which function as nouns; and infinitives, which have noun-like, adjective-like, and adverb-like functions. Each of these kinds of verbals is also used in various common constructs; for example, the past participle is used in forming the perfect aspect (to have done).
参考资料：http://en.wikipedia.org/wiki/Non-finite_verb
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