"My grandfather can speak English."
Although I give few grudges to duolingo for absconding on cultural education, there is a specific way to refer to one's relatives context dependent in Chinese. Speaking to them, about them, or with them in either a public, private, or professional setting has variation called honorifics, 敬辞. Posted for anyone interested.
The meaning could be akin to 'anticipate' as in both 'one is able [to perform an action]' or 'one is about to [perform an action]' hence 'one anticipates doing something' in a concrete or objective manner.
Words and phrasings are available for the strict modal ability 能 or future intepretations, as in 'one is currently learning to be able to [in future] perform an act' or 'one is reluctant to perform an act yet will commit to doing so regardless'.