The Big Four
Certain core concepts are extremely important for students to master completely. You can't do math very well if you don't know the difference between "+" and "-". You can't do Spanish well unless you know the core ideas either. The Big Four are the most common, important, and tricky verbs in the language.
I always say that students should know these items "backwards, forwards, and in their sleep." They will be given multiple opportunities to pass both sets of tests (English to Spanish and Spanish to English). Their course grade will be an Incomplete (I) until they have mastered both tests at 100%. Spelling (including accent marks) counts.