What causes white lines on teeth, i am a grinder and i grind a lot on a invisalign type of retainer.

Decalcification?: The white lines may be decalcification lines if they occur at the edge of your retainer, if plaque tends to accumulate there and then the acid produced by the bacteria start the decalcification process. For an accurate diagnosis or evaluation, you need to actually see your dentist in person.

Orthopedic Dentist: The cause of bruxism needs to be discovered. Most dentiist make a bite appliance for you to wear to keep from grinding away the teeth. This is only treating the symptom and not the cause. The better option is to consult with a dentist that is familiar with treatment using an orthopedic appliance and TMJ treatment modalities.

Plaque & acid: I assume you are asking about lines or scars on teeth that can be produced by bacterial plaque left by poor oral hygiene and other consumed acids like soft drinks. Bruxism or grinding is not the cause of those types of lines.

Usually not: It is hard to say without a picture. Generally white spots are cause by ingesting too high a concentrations of fluoride, front teeth developed between age 3-7. Most likely due to well water or using a big dollop of toothpaste on your toothbrush (as most ads on tv advertise). You only need a pea size amount. White spots can be corrected, see a dentist for microabrasion treatment.

Craze lines: These fracture (craze) lines are like superficial cracks in in a porcelain plate...Nothing may happen, but be careful! and always wear your night guard when sleeping.

Lots of reasons: Your description of "white lines" isn't very informative. Can be decalcification, fractures, abfractures, fluorosis, stain, or many other causes. Please see your Dentist to discuss possible causes. If your occlusion is less than optimum, ask for referral to an Orthodontic Specialist.