Modals and passive voice

Modal verbs such as can, could, may, might, must, should and ought to add meaning to the main verb, and they are always followed by the bare infinitive (the base verb with no 'to' and no -s): 'She can swim', not 'She can to swim' or 'She cans swim'. We use can/could for ability, permission and requests, may/might for possibility, must for strong obligation or for a confident deduction, have to for an obligation that comes from outside, and should/ought to for advice. Be careful with negatives: 'mustn't' means something is forbidden, while 'don't have to' and 'needn't' mean there is no necessity to do it. The passive voice is formed with the verb 'be' plus the past participle (V3): in the present simple it is am/is/are + V3 ('English is spoken here'), and in the past simple it is was/were + V3 ('The house was built in 1990'). We use the passive when the action or the result matters more than the person who does it, and we can name the doer with 'by'. When we change an active sentence into a passive one, the object of the active sentence becomes the subject of the passive sentence. For example: active 'Workers built the bridge in 1995' becomes passive 'The bridge was built by workers in 1995'.