This apparatus can/is able to detect earth tremors. - Ability in the present, A lot of chemicals that you could/were able to buy in shops in the past are now on restricted sale. - Ability in the past (general), In the end, the college was able to raise enough money to buy new science equipment. - Ability in the past (one specific occasion), This new TV programme about astronomy may/might/could be interesting. - Possibility, The presenter may have/might have forgotten that most viewers know very little about physics. - Possibility in the past, I can't use my smartphone because the battery is flat. - Impossibility in the present, The psychologist couldn't have won the prize without the work done by her team. - Impossibility in the past, She goes abroad on holiday several times a year, so she must have a good income. - Certainty (with a logical evidence), This chemical reaction shows there can't be/can't have been oxygen present. - Certainty that is not/was not the case, You must all wash your hands thoroughly after working with chemicals - Obligation (coming from the speaker), You have to register in advance if you want to attend the lecture. - Obligation (coming from elsewhere), You can/may watch the student presentations as long as you don't interrupt. - Permission, When I was a child, I could/was allowed to stay up late to watch educational programmes on TV - Permission (as a general rule) in the past, The children were allowed to stay up late last night to watch a TV programme about electronics. - Permission (on a particular occasion), You can't/mustn't go into the main hall because there's an exam going on. - Prohibition , You should (have)/ought to (have) read this book if you want to really understand the subject. - Recommendation (present and past),

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