Summarize this article:

Even if you are just driving straight ahead on an open highway, it's important to know what is going on behind you. This means you should be looking at your rearview mirror every 5-8 seconds.  This will keep your understanding of the traffic conditions behind you up to date. It will also allow you to keep track of cars that are passing you, and even cars that are acting sporadically or dangerously behind you. You should look in the mirror before you stop or start, pass a car, turn, switch lanes, pull over, or make any other adjustment to your position on the road. Basically, if you want to make any movements, other than straight ahead, you should check your rearview mirror before doing it, so that you can make the change safely. When changing lanes, you use a combination of your rearview mirror and your side mirror to see if where you are moving is clear, as well as turning around to see if there is anyone in your blind spot. The combination of all of these checks will make switching lanes safer. The rear-view mirror is especially important when you are moving your vehicle in reverse. The mirror will help you to know the course is clear and that you can move without coming into contact with another car, person, or property. When moving in reverse, it is also important to rely on things other than the rear-view mirror. Check your side mirrors and turn around and look behind the car with your own eyes in order to assess the situation. After all, it is always better to be safe than sorry. The rearview mirror can give you a lot of good information when trying to get through a stressful and busy driving situation. For example, look in your mirror to assess whether moving in reverse will help you get out of a tight space or just lock you into an even more tenuous position. Before you brake hard, take a quick second to assess whether any cars are too close to your rear end to stop. If that is the case, consider adjusting your lane or your braking speed if you can. If you can't, then knowing a car behind you may hit you will give you a second to brace for an impact.  If you are looking in your rearview mirror at regular intervals already, you may already have a good idea if a vehicle is close behind you. This will help you to make a quick judgement about the following distance and if a vehicle will have time to stop before hitting you. Being good at judging the following distance of vehicles behind you should help you adjust your braking measures in order to prevent a rear-end collision from happening.

Summary:
Check your rearview mirror on a regular basis. Look in your rearview mirror before adjusting your position on the road. Use your rearview mirror when backing up. Look at your rearview mirror when navigating heavy or irregular traffic. Look before you make a quick stop if you can.