| Input is a vital element in the complex process of second language acquisition (SLA). Krashen's Input Hypothesis emphasizes the significance of comprehensible input and its incomparable position in SLA. However, it is still controversial about whether comprehensible input directly influences second language (L2) acquisition. Some scholars argue that it is comprehended input rather than comprehensible input that is effective in L2 acquisition, whereas others suggest that the role of comprehensible input has been exaggerated and the importance of output is neglected in the process of SLA as well.The researcher of the current study reviewed the importance and contributions of comprehensible input to SLA, targeted at university level L2 learners, employed a pretest-posttest-delayed posttest design to investigate the effects of different types of comprehensible input– lexically premodified input (simplified modifications, elaborative modifications, and textual enhancement), interactionally modified input, and modified output–on L2 reading comprehension, vocabulary recognition (acquisition) and vocabulary production (acquisition).The participants of the current study were 255 non-English major freshmen who were randomly chosen from the intermediate level classes and assigned into one control group and five experimental groups. The researcher offered the subjects different treatments to investigate the effects of comprehensible input on L2 reading comprehension and vocabulary acquisition, and monitored the procedure in the mean time. The participants were given posttests to examine their comprehension of the reading material, ability of vocabulary recognition immediately after the treatment, and additionally delayed posttests to test their vocabulary acquisition. SPSS (15.0) was employed in this study to analyze the collected data.The findings of the study were: (a) typographically enhanced input aided L2 reading comprehension, whereas both simplified input and elaborated input impeded L2 reading comprehension; (b) premodified input and interactionally modified input facilitated L2 reading comprehension while modified output hindered L2 reading comprehension; (c) both elaborated input and enhanced input aided L2 vocabulary recognition with the relatively similar effects; (d) premodified input, interactionally modified input, and modified output facilitated L2 vocabulary recognition, and furthermore modified output significantly outperformed other input-types on immediate vocabulary recognition; (e) both elaborated input and enhanced input aided L2 vocabulary production along with the better effects of the former; (f) premodified input, interactionally modified input, and modified output facilitated L2 vocabulary production, and furthermore interactionally modified input significantly outperformed others on vocabulary production; (g) the facilitated effects of comprehensible input on SLA may owe to learners'noticing and attention to the target language features; (h) L2 learners can acquire vocabulary from reading, and the acquisition percentage of adjective is the highest, followed by phrase, verb, and noun; (i) the recognition of the target forms was not significantly relevant to the frequency category and the band of CET.In conclusion, this dissertation verifies the positive role of comprehensible input in SLA, discusses the implications of comprehensible input in ESL and EFL teaching, summarizes the limitations of the current study, and finally proposes suggestions for future studies. It suggests that as long as L2 learners are exposed to a large amount of input, target-language acquisition will finally occur. In addition, negotiation, textual enhancement and word-focused activities definitely facilitate L2 reading comprehension and vocabulary acquisition. Subsequent studies should trace the path of input study and more attempts should be made to investigate the interactive effects of different types of input, and the effects of noticing and attention on SLA.
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