"My grandfather can speak English."
But doesn't 会 means "will ". Isn't 我爷爷说英语 Correct ?? Thank you in advance. ( 谢谢 )
Yes, though it is common in mainland China or related communities that this type of sentence is spoken.
You can also express the sentence without using 会, such as 我爺爺可以講國語, where 可以 expresses "can". This is more meaning-strict than the sentence the team gave.
会 in this case means "can" in terms of a learned ability. Or something like that. So 会说 means "can speak"
That would mean "My grandpa speaks Chinese" without a mention that he learned it at some point. 会 also means "can, know how to".
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.