posted by y912f .
Cuando escales montañas, quiero que _____ mucho cuidado.
When you climb mountains, I want you.......very careful.
This is the idiom "to be careful" = tener cuidado
Do you see the main verb "quiero" = I WANT, which is imposition of will. You never know if this person will be careful or not. This is a typical use of the Subjunctive.
Can you pick it out then?