Here is a worksheet on differentiating between the verbs flounder and founder, informed by Paul Brians’ book Common Errors in English Usage (which he makes available at no charge on the Washington State University website). This worksheet contains a single-paragraph reading from Professor Brians’ book, with ten modified cloze exercises. However, since it is a Microsoft Word document, you can manipulate it to meet the needs of your classroom.
These are two intransitive verbs (founder has a transitive use, “to disable (an animal) especially by excessive feeding,” of which I was unaware, clearly because this word is seldom used in American English to convey this ghastly meaning) which are frequently confused. Once again, Professor Brians summarizes them elegantly: “If you’re sunk, you’ve foundered. If you’re struggling, you’re floundering.”
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.