What linguistic feature is often challenging for ESL learners?

Idiomatic expressions often present substantial challenges for ESL learners due to their figurative meanings, which may not be directly understood from the individual words that comprise them. Unlike literal phrases, idioms can convey meaning that is culturally based or context-specific, making them difficult for non-native speakers who are still in the process of grasping the nuances of a new language. For example, the expression "it's raining cats and dogs" does not literally refer to animals falling from the sky; instead, it means it's raining heavily. This reliance on cultural understanding and contextual cues makes idiomatic expressions particularly tricky for learners who may translate phrases word-for-word from their native language.

In contrast, complex sentence structures, while challenging, can often be broken down and analyzed more easily than idiomatic expressions. Simple vocabulary tends to be accessible for learners, as it generally involves words that are commonly encountered and understood. Pronunciation, although a significant factor in mastering a language, typically deals with phonetics and sound rather than the idiomatic use and meaning, making idiomatic expressions uniquely difficult for learners at all levels of fluency.