Sure! Here's the completed text with the appropriate verbs filled in:

1. **Can** you stand on your head? - I **could** when I was at school but now I **can’t**.
2. When I was a child I **could** understand adults but now I am an adult I **can** not understand children.
3. When the fog lifts we **will be able to** see where we are.
4. I used to **be able to** stand on my head but I **cannot** do it now.
5. The swimmer was very tired but he **was able to** reach the shore before he collapsed.