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Modal verbs are used to make precise statements and for that reason are very common in scientific and technical texts. Modal verbs have no meaning by themselves. They arealways used in conjunction with main verbs to help them express ideas (about the past, the present or the future).

Subject + Modal verb + Main verb + Complement
Never add -s, -es, -ed, or -ing to thesecond verb. Modal verbs behave in a different way from ordinary verbs. They don’t have -ing form, and they don’t add -s in the third person singular (he, she, it). There are several categories ofmodal verbs. The most commonly used are: Ability: can, may. • You may do this in order to get the best possible performance. • Civilizations cannot prosper without access to clean drinking water andsanitation services. Obligation: must, ought to, should. • Students in that course should attend every lecture. • Students must enter the examination room quietly and may not bring into it any electroniccommunication devices. A student needing to speak to the proctor should so indicate by raising his or her hand. • Assets must always equal the sum of liabilities and shareholders' equity. • We mustfind ways to meet water demands with minimum ecological disruption. Possibility / probability: can, could, may, might, will, would. • Wastewater may pose a threat to human health or the environment. •Ongoing shifts in climate would affect the health and welfare of millions of people around the world. • Higher temperatures might trigger severe flooding in some places and severe drought in others. •Coastal flooding and storms will raise the toll of deaths, injuries and related diseases.

The passive voice is used to describe situations, methods, procedures, processes, etc. Itinvolves the following elements:

Object + Verb to be + Main verb (past participle) + Complement Tense Present Present perfect Past Past perfect Future Future perfect Object The car/cars The car/cars...
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