Must, Can't structure


i found on my grammar the Must, Can’t Structure, and sometimes they are followed by the past participle and sometimes by the gerund, without explain why. For example:

“Jane walked past me without speaking. She can’t have seen me”
“Tom walked straight into a wall. He can’t have been looking where he was going”

So, why in this case we use differently past participle and gerund?


The difference is between the Present Perfect (have seen) form and the Present Perfect Continuous (have been looking) form. You wouldn’t use the continuous form with ‘see’, which in itself describes a continuous state but it is possible with ‘look’ because that verb is not in itself continuous only in its meaning/state.