The book, written by Michael Swan, aims to address questions raised by relatively advanced learners of the English language, such as "When do we use a past tense with a future meaning?" and "Do I say 'big', 'large', or 'great'?"

"Swan's ability to explain grammatical rules simply and clearly has won accolades from appreciative readers who value the way Practical English Usage provides answers to even the most challenging of students' questions," said Marc Goozée, publishing manager for grammar at Oxford University Press.

"The app makes it easy to navigate the content in a number of ways: by using the index, the thematically-organised Contents Overview, or the A-Z list of entries.

"The app is fully searchable, so you can also key in the language point you want to look up.

"Practical English Usage has been a classic reference tool for many years, and we expect it to become the must-have app for all serious language learners and teachers," he said.

It also includes notes on differences in British and American usage.

Practical English Usage costs $41.99 from the iTunes Store.

An Android version is planned.