The two verbs (in each of your example sentences) are dealing with separate situations, so they might be either present or past.
For example, in your second sentence, if their ability to show still exists, then "can" would be appropriate. If you're talking about an ability that they had in the past, then "could" would be the better choice.
Note that the past tense for the second verb is often used to express a current state, as well as a past state.
1. I forgot you were at work (when I called your home earlier today).
2. I forgot you are at work (when I dialed your cell phone number just now).
But "were" is also frequently used in situation #2.
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