Can, Could, May
Can, could, and may are modal verbs (See Modal Verbs), which are a form of auxiliary verbs (See Auxiliary Verbs). They are used before other verbs to show ability or possibility.
In statements (see below about questions), can is used to talk about the present or the future (ex. I can speak Japanese). Could is used to talk about the past (ex. She could walk when she was only 2 years old.). Can and could are often used to make requests. Can is more informal (Can you pass me the salt?) Could is more formal and polite (Could you take care of our dog while we are on vacation?) Can and could are also used sometimes to ask for permission. Like with requests, can is more informal and could is more formal. Even more formal is may (ex. May I use the restroom?)
In fact, some people believe that using can and could for asking permission are not correct. However, in recent years, it has become common. Can is also used to offer help. Could
is not usually used in this way.
Since can and could are modal verbs, they are also auxiliary verbs and sentences with them follow the pattern of other auxiliary verbs.
Subject + can/could/may + verb
Ex. I can play basketball.
Can/could/may + subject + verb
Ex. Can you swim?
When using modals like can and could, to is not used before the infinitives (dictionary verb forms) like with other kinds of verbs. ex. I can play piano. (NOT, I can to play piano)
They also do not use ~s before them when the subject is 3rd person singular. ex. He can dance. (NOT, He cans dance).
Questions and negatives do not use do with modals. ex. Can you speak Russian? (NOT, Do you can speak Russian?) There is no infinitive or past participle form of modals. Instead, be able is used instead. ex. I hope to be able to drive soon. (NOT I hope to can to drive soon.) ex. I have not been able to finish. (NOT, I haven’t could finish.)
Can/could is usually preferred when followed by passive infinitives (be + past participle) ex. It can’t be understood. (more natural than It is not able to be understood.)
When talking about future possibilities based on currently abilities, can is used. However, when the ability or skill will be gained in the future, will be able is used. ex. I am a good swimmer so I can swim in the race tomorrow. ex. I will be able to swim after taking the swim lessons.
May is also used to talk about possibilities.
See May, Might. See also Able, Unable, Modal Verbs, and Auxiliary Verbs.