What types of construction, syntactic elements and vocabulary make language easy or difficult for learners of English to understand?
Are there significant differences between the difficulties encountered in spoken and written language for learners? (And therefore the kinds of guidelines we might want to give teachers, and helpers of English language learners)
This might relate to written or spoken instructional and presentational language (in classrooms or in textbooks), or for example posts on ELL, where we might consider a lot of the answers to be either instructional or presentational .
This question is really important for teachers and people who want their posts to be accessible. It's also important for teachers who want their language presentations and classroom language to be understood by learners.
I'm interested in advice supported by evidence, and not looking for opinions or (primarily for) anecdotal answers.