Resource availability

Hi,
I have a question about setting up task: if i am setting task and assigning it to user without specifying the start date but with predecessor set and the duration specified and constraint as soon as possible I would expect this task will appear at the first available date for the assigned resiource but in Gantt chart I can see the link from predecessor to unknown time in past.
What I am doing wrong?
Another question: how to set up critical path?

Regards,
Evgueni

What you mean by replan the project?

This have moved the predecessor start date to the beginning of the project not the date it have been scheduled

Ok, if I understand it right, I need to recalculate project FROM THE DATE OF PREDECESSOR start date. Am I right?

Regards,
Evgueni