MODALS, SEMI-MODALS AND SIMILAR VERBS: PAST CATEGORY MODALS + base form/perfect base form invariable work as auxiliaries Ability Possibility Probability COULD + base form SEMI-MODALS & SIMILAR VERBS + base form variable do not work as auxiliaries > I couldn t speak French when I was 12. BE ABLE TO (past tense) + base form > I wasn t able to understand the figures. COULD + base form > I couldn t enter: it was forbidden. COULD + perfect base form > We could have got a better solution if we had asked. MAY/MIGHT + perfect base form > The surgery may/might have been delayed because of the emergency, but it wasn t. Advice SHOULD/OUGHT TO + perfect base form > What should we have done with this complaint? We ought to have given them a refund. Permission Prohibition COULD + base form COULDN T + base form > Could you cancel the order? > Were we allowed to ask for some compensation? > You couldn t use the photocopier because you didn t have Lack of obligation NOT BE ALLOWED TO (past tense) + base form > The employees weren t allowed to ask for a pay rise. your badge with you. Obligation BE ALLOWED TO (past tense) + base form HAVE TO (past tense) + base form > We had to fill in the form online. NEEDN T + perfect base form > They needn t have replied, there was an automatic service DID NOT NEED TO + base form > They didn t need to reply, there was an automatic service that did it. (They replied because they didn t know it was unnecessary) doing it. (They didn t reply because they knew it was unnecessary) NOT HAVE TO (past tense) + base form > They didn t have to send the application yesterday because they put off the deadline. Necessity HAVE TO (past tense) + base form > B efore sending the goods back, you had to ask quality control first. NEED TO (past tense) + base form > You needed to enclose your medical history, that s why they had sent you that form. Deduction MUST + perfect base form > The carrier must have had some problems, it isn t late, usually. CAN T + perfect base form > It can t have been our CT scanner to break down, it s new! 390 extra