The present chapter investigates how the quantity of input and the linguistic characteristics of the Heritage Language (HL), Spanish or Russian, impact the comprehension of English Subject-Verb Agreement (SVA) (Study 1) and the production of English vowels (Study 2). In both HLs, SVA exhibits more contrastive forms than in English. Study 1 demonstrates that this HL characteristic facilitates the comprehension of English SVA by bilingual preschoolers, more so than the percentage of exposure to English. In contrast, both HLs exhibit a smaller and phonetically different vowel inventory compared to English. Study 2 demonstrates that the exact vowel inventory of the HL combined with amount of English exposure helped to explain the acoustic characteristics of the productions of bilingual Spanish-English and Russian-English preschoolers. These results have clinical and educational relevance.