Modal Verbs: Ability & Advice

Modal verbs are special helping verbs that we use before a main verb to talk about ability, permission and advice. We use 'can' to talk about ability in the present (I can swim) and to ask for or give permission (Can I go out? You can sit here), and we use 'could' for ability in the past (When I was five, I could read) and for polite requests (Could you help me?). We use 'should' to give advice or to say that something is a good idea (You look tired. You should rest). A very important rule is that after a modal verb we always use the bare infinitive, which is the base verb with no 'to' (She can swim, NOT She can to swim). Modals do not add -s in the third person, so we say 'He can play', not 'He cans play'. We make negatives by adding 'not' to the modal: cannot/can't, could not/couldn't, should not/shouldn't (You shouldn't eat so much sugar). The same modal form is used for every subject, so it never changes. For example: 'You should drink water, but you can't drink the river water because it is dirty.'