My English teacher told me I needed to work on my vocabulary.

This joke expertly employs both wordplay and irony to land its punchline. The setup suggests a common classroom scenario where an English teacher points out a student's need to expand their vocabulary. The humorous twist comes with the punchline, "I was speechless," which is a direct, albeit exaggerated, consequence of having a limited vocabulary. It's ironic because the student's reaction perfectly demonstrates the very problem the teacher identified, proving the point in a literal and unexpected way.

The humor also stems from the relatable experience of being a student in an English class, where vocabulary lessons and assignments are a staple. Many can recall the pressure to learn new words or the frustration of being unable to articulate a thought due to a lack of suitable terminology. Being "speechless" can also be a common reaction to receiving feedback, making the punchline a double-edged sword that highlights both the student's literal word shortage and a perhaps feigned shock at the teacher's advice.