it is the academic language of middle and high school classrooms and texts that prove most difficult for ELLs and in spite of the fact that ELLs—and their classmates—need between 12 and 14 exposures to a word and its meaning, across multiple contexts (different texts, classroom discussions, writing activities), in order to gain deep understanding of a word.
Francis, David & Rivera, Mabel & Lesaux, Nonie & Kieffer, Michael & Rivera, Hector. (2006). Research-Based Recommendations for Instruction and Academic Interventions. Practical Guidelines for the Education of English Language Learners. Book 1 of 3. Center on Instruction.
"Table 11: Error types that improve significantly between C1 to C2 levels" Verb Agreement
When the verb is correct and the form of the verb is valid but wrong in the context because it does not agree grammatically with its coordinates, it is a Verb Agreement error.
Something which grows in popularity are the solar cells. (is) The world have changed. (has)