A modal auxiliary verb is used to modify the mood of a verb. <br /><br />Can:<br />“Can” is used to express ability, willingness, permission, <br />or possibility. <br />The negative of can is “cannot” or the contraction “can’t”. <br /><br />Example:<br />I can cook dinner. <br />In this example “Can” expresses the ability to cook dinner. <br /><br />Could:<br />“Could” has at least three functions. <br /><br />First, it can replace “can” and give the phrase a more conditional tone. <br /><br />Second, it can function as the past tense of “can”. <br /><br />Third, it can function in the same way as “might” or “may”, suggesting that something is a possibility. <br /><br />The negative of could is “could not” or the contraction “couldn’t”.<br /><br />Example:<br />You could have been an astronaut if you wanted to.<br /><br />In this example “Could” expresses the conditional ability to be an astronaut in the past. <br /><br /><br />
