can  　 〜できる
   同意語 be able to
   過去形 could     未来形 will be able to

    You  can         speak English.
  = You  are able to speak English.　　話すことができる

    You could        speak English.
  = You were able to speak English.　　話すことができた

    You will be able to speak English. 話すことができるだろう