"they can run fast"

• be able to through acquired knowledge or skill

  "I can speak Italian"

• have the opportunity or possibility to

  "there are many ways holidaymakers can take money abroad"

• used to express doubt or surprise about the possibility of something's being the case

  "he can't have finished"

• used to indicate that something is typically the case

  "antique clocks can seem out of place in modern homes"

"you can use the phone if you want to"

"can you open the window?"

• used to make a suggestion or offer

  "we can have another drink if you like"