This study examined English-acquiring 20- and 24-month-olds’ ability to detect subject-verb dependency. Twenty-four-month-olds showed a significant preference for grammatical sentences over ungrammatical sentences in which ungrammaticality was cued by a pairing of a singular subject with the verb “are” but not when it was cued by a pairing of a plural subject with the verb “is”. However, 20-month-olds did not show a preference in either condition. Another group of 20-month-olds were examined on their ability to detect a non-adjacent dependency in which a prepositional phrase was inserted in between the dependent elements. They showed no preference for either the grammatical sentences or the ungrammatical sentences. The result of this study revealed that it is not until about 24-months that children acquire an understanding of the structural properties of the relationship between the subject and the verb “to be” and this knowledge at first is limited to a singular subject.