When should you use 'can' and when should you use 'could'? 'Could' is the past tense of 'can' and is often used for simply indicating past tense. However, 'could' is also used for suggestions and requests. 1. 'You could try a new exercise routine.' (Suggestion) 2. 'Could you lend me your book for a few days?' (Request) Note: 'Can' is also correct for this question as it is used often in everyday English, but 'could' is more polite or more tentative. 'Can' is used for giving or refusing permission to do something. 1. 'Yes, you can borrow my book.' 2. 'No, you can't borrow my book.' Note: some people consider this usage of 'can' as incorrect and say that instead of 'can' you should use 'may'/'may not' (which is mostly reserved for very formal situations). These are only a few examples of when to use 'can; and 'could'. For more examples refer to this helpful article: https://blog.oxforddictionaries.com/2013/07/01/can-or-could/ For more about modal auxiliary verbs (such as 'may' and 'might'), refer to this: https://blog.oxforddictionaries.com/2013/04/05/may-or-might-whats-the-difference/ --- Feel free to check my profile for intensive English courses and test prep.