This article analyses the factors influencing the process of input-intake conversion and focuses on the effect of “reduced forms” on English as a Second Language (ESL) learners’ listening comprehension. The Input Hypothesis, the Noticing Hypothesis, the input-intake relationship, and the factors influencing the input-intake relationship are critically reviewed and analyzed. The empirical study of Brown and Hilferty [1] is reviewed and discussed to show reduced forms’ influence on ESL learners’ listening comprehension. The results prove that integrating reduced forms into ESL lessons is both necessary and meaningful for improving students’ listening comprehension ability.