This study utilized a paired priming paradigm to examine the influence of input features on case assignment in typically developing English-speaking children. The Input Ambiguity Hypothesis (Pelham, 2011) was experimentally tested to help explain why children produce subject pronoun case errors. Analyses of third singular "-s" marking on elicited verbs with nominative and non-nominative subjects were used to test claims of the Agreement Tense Omission Model (ATOM; Schutze & Wexler, 1996). The first aim was to determine whether priming children with input featuring case contrasting pronouns (i.e., "I-me" and "we-us") versus non-contrasting pronouns (i.e., "you-you" and "it-it") would differentially influence children's subsequent subject pronoun productions. It was hypothesized that children would be more likely to make subject pronoun case errors when primed with non-contrasting pronouns versus contrasting pronouns. The second aim was to determine whether case contrast features in the input might also influence finiteness marking, particularly for third person singular "-s" forms. Following the predictions of ATOM, it was hypothesized that children would be less likely to include third person "-s" forms on verbs with non-nominative versus nominative subjects. The third aim was to determine if any other spontaneous or elicited language features were associated with performance on the experimental task. Thirty typically developing English-speaking children (ages 30-43 months; 15 female) participated. Utilizing a within-subjects design, the children completed both conditions of a paired priming task, counter-balanced across participants. In the contrasting pronouns condition, children were primed by imitating a pair of simple transitive phrases containing examples of case-contrasting pronouns (e.g., "Dad drives us and we drive Donald Duck"; "Mom drives me and I drive Pluto"). Then the children immediately completed a pair of sentences to elicit a third person singular subject pronoun (e.g., "Dad drives her and..."; "Mom drives him and..."). In the non-contrasting pronouns condition, children were primed with the same sentence types except the pronouns did not have case contrast (e.g., "Dad drives it and it drives the pig"; "Mom drives you and you drive the duck"). The same subject pronoun elicitation sentences were used in both conditions. Spontaneous language samples, a pronoun elicitation task, and standardized language measures were also administered to all participants. Results indicated that children were more likely to produce "him-for-he" and "her-for-she" errors when primed with non-contrasting pronouns ("it-it" and "you-you") versus contrasting pronouns ("I-me" and "we-us"). When children used non-nominative pronouns, they were equally likely to use or omit third person singular "-s" (e.g., "her hug Elmo" and "her hugs Elmo"). When children used verbs marked for third person singular "-s," they were equally likely to use nominative and non-nominative case subject pronouns (e.g., "him hugs Elmo" and "he hugs Elmo"). Results of this study provide counter-evidence to claims made by ATOM and suggest a need for a theory of tense/agreement acquisition that accounts for influences of both hard-wired universal grammar principles as well as input-driven factors. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]