What is the future tense of the word "can"?
The future (simple) tense of verb "can" is "will be able to"
Can is a defective verb, which means it does not have all valid forms used in different tenses. For example it is not used in Future tenses, but it can be replaced with a phrase "to be able to" which has all forms and can be used in any tense (excluding continuous/progressive tenses which are not used for modal verbs can, must, should etc.).
The forms of phrase "be able to" consist of appropriate form of verb "to be" + "able to". Examples:
Future Simple: will be able to
Present Perfect: have/has been able to
Future Perfect: will have been able to