Educators and researchers have been interested in supporting sentence-level language comprehension for struggling readers, but it has been challenging to research. To investigate the properties of sentences that might be useful targets for future research in instruction and assessment, we coded several features of the items in a computer-adaptive scale of sentence comprehension. The Syntactic Knowledge Task is designed for students in Grades 3-10. We then explored how the features of the sentences were related to the item's difficulty value to determine which aspects of sentence-level language made sentences more and less challenging for students across a range of development. We found that genre, words that represent a logical connection, number of idea units, long words, and words on the Academic Word List were significantly associated with item difficulty. Implications for understanding students' sentence-level language development are discussed.