The Incomplete Acquisition Theory recognizes that complete acquisition of L1 takes place throughout childhood, stretching into the school-age period.[12] Therefore, heritage language learners, who typically switch to the dominant culture’s language when they enter school and, consequently, experience a decline in input in the heritage language, do not completely acquire their first language.[16] As heritage language learners are rarely schooled in their L1, they are often orally proficient but illiterate or have underdeveloped written comprehension and production in the heritage language.[11] In treating heritage language learning as interrupted L1 acquisition, in which the learner has access to Universal Grammar, it is expected that heritage language learners have knowledge of concepts found in early stages of L1 development, such as binding constraints, wh-movement, and aspects of lexical semantics, and struggle with grammatical concepts that require sustained exposure and practice in school-aged children, such as specialized vocabulary and uses of the subjunctive in Spanish.[12][14]