To use Modal Verbs correctly is a must for every English language learner. Here is an app which offers detailed grammatical notes on English modal verbs' usage, and 5 practice and quiz activities to practice them.
The first practice activity, with 50 items, tests your ability to recognize the function a modal expresses. The other 4 practice activities, with 50 items each, test your ability to use modal verbs correctly.
The first quiz, with 50 items, tests your ability to recognize the function a modal expresses. The other 4 quizzes, with 50 items each, test your ability to use modal verbs correctly.
It's designed for English language learners (ELL students) regardless of their levels of English language proficiency.

TESL/TEFL teachers may want to recommend it to their students.

It has been tested successfully on a variety of handheld devices.