There are a number of reasons EFL students have a hard time understanding idioms: In part, language learners find it difficult to guess the meaning of expressions that may not follow traditional grammar rules and that don't add up to the sum of their parts. Try this classroom activity to help your EFL students attain idiomatic comprehension and proficiency. 

The study discussed in this article investigated first language effects on listener judgment of comprehensibility in second language speech. Findings show that comprehensibility was associated with several linguistic variables, and that accentedness was linked to pronunciation. Download the full article and results here.