References: 

Owens, R. E. (2016). Language disorders in children. In Language disorders: A functional approach (7th ed., pp. 354-357). Pearson. 

Riches, N. G., Tomasello, M., & Conti-Ramsden, G. (2005). Verb learning in children with SLI: frequency and spacing effects. Journal of speech, language, and hearing research : JSLHR, 48(6), 1397–1411. https://doi.org/10.1044/1092-4388(2005/097)