How is the availability of schedules determined when scheduling a tutoring lesson?

The availability of dates and times vary for all tutors. Each tutor defines his or her availability, you may notice there are some tutors with more dates available than others.

Why does the availability of tutors is determined by time lapses depending on the subjects?

This is because each subject has a duration predetermined. This means that if a tutor is available from 9 to 11 AM and the subject selected has a duration of 30 minutes, only the time lapses of 9:00, 9:30, 10:00 and 10:30 will show as available for requesting a lesson.

Why does a determined hour is no longer available for scheduling in the available time of a tutor? 

If a determined hour is no longer available it means that the tutor has another lesson already scheduled at that time. Tutors are only able to teach one lesson at a time.