1. Tell me when to meet.

2. Tell me when I should meet.
3. Tell me when you should meet.

(What does #1 mean, #2 or #3?)

4.She hurried up only to be late for school.

5. She hurried up, but she was late for school.

6. She hurried up, but was late for school.
(Does #4 mean #5 or #6?)

#1 is adequate, but 2 and 3 are incomplete. You don't state whom each person is going to meet.

These are complete:

Tell me when to meet you.
Tell me when I should meet you.
Tell me when you will meet me.

#s 4-6
All three of these sentences mean approximately the same thing.