My Invisalign retainer has cracked to the point that I can separate it with my fingers if I tried, can I keep wearing it safely?

hi, I can understand and appreciate your concern. Teeth have a tendency to move back towards their initial position very quickly. If you've your retainer broken, don't wear it as its not going to serve the purpose. Please see your dentist ASAP and get a new retainer made immediately before the teeth move. 

Teeth can shift positions within a few days and will continue to move over time. How quickly or how much they will move is different from person to person. Consistent retainer wear is the only way to keep teeth in the position they were in following orthodontic treatment. Upon treatment completion all of our invisalign patients leave with 4 sets of retainers. Typically a retainer will last 1-2 years before it looses integrity and isn't effective at maintaining teeth position. Once a retainer is cracked or has holes it needs to be replaced. I recommend you call your dentist for and have new retainers made. If your cracked tray goes in with ease you can wear it while your new retainers are being made. If the retainers do not go back in easily don't force them and be sure to bring them to your appointment. Good Luck