MODAL VERBS

Possibility

Can, could and might all express possibility. Could and might are similar in meaning. Both are used to say that something is possible.

• I might see you tomorrow.
• I could be late for the meeting.

However, in the negative only might is used.

• They mightn't go to the concert.
  NOT: They couldn't go to the concert.

Can is also used to express possibility, but it has a slightly different meaning.

• Going on vacation can be expensive.
• Using a keyboard can cause RSI.

Can is used here to suggest that an action or situation is possible sometimes, but not always.

Compare these two sentences.

Here the present simple is used for a situation that is always true:

• It's cold in the Arctic in the winter.

Here can is used to suggest that a situation is possible sometimes but isn't always true:

• It can be cold in San Francisco in the summer.