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Because of limb length varying so much it can be difficult to explain in text how to assume the starting position in the deadlift. Furthermore advanced lifters will find their own way. Here’s a good starting point:
For many this will immediately result in a decent starting position. Some will sit too low, in that case raise their hips and pull the bar closer to the chins. If the back is too rounded, a slight simultaneous touch on the lower back and front of the shoulder will help flatten it out.
If the toes are pointing straight ahead, point them out slightly. Never point the toes inward. If it still doesn’t help “screwing the feet” can help. A healthy dose of Romanian deadlifts and Sumo deadlifts can also be beneficial.
Pretty much always a lack of back strength. The upper back should control the bar. A significant amount of work on that area and practicing the deadlift with only manageable weights will help. A specific style of deadlifting, with a controlled lowering can be useful in addition to regular deadlift training.
This usually only becomes a problem for very strong lifters. Most will not need specific grip work but if for some reason you're one of them you can utilize everything from grippers to fat bars, doing chins on ropes, etc. Another idea is to hold each final rep of a deadlift set for several seconds.
Note that rounding of the upper back is not considered an issue. It’s a perfectly valid way to lift as long as the lifter straightens it at the top of the lift. The lower back is a different story.
First make sure the lifter use correcting breathing and tension in the core as explained in section ‘1.4. Power breathing’. Second, ensure that the lifter pulls the slack out of the bar before initializing the lift. If it still persist the lifter will have to lower the weights and practice with as low weights as necessary to keep the lower back flat and progressively add weight from there. With advanced lifters who have built up a significant amount of strength with a rounded lower back this can take a lot of time.
Rounding the lower back in the opposite direction, which is basically what happens when you overextend a lockout, is dangerous just as rounding it in the lift is. First simply make the lifter aware of it. If it doesn’t fix the problem put your hand so the lifter’s upper back hits it where it should stop. A useful cue is “attention!”, as in the military, meaning stand straight. Telling the lifter to crush a walnut between their butt cheeks also helps.
Either it's simply too heavy or the grip is too weak (even if the lifter doesn't feel it). More often than not it’s just a matter of not attempting the weight again until the lifter have gotten stronger with less weight.
This is the most common sticking point for conventional deadlifters. Practice driving the hips through rather than just pulling. If the lower back has rounded this will be very difficult or impossible.
Hips are not used enough. Drive them hard and don't just pull. If the lower back has rounded significantly at this point it won't work. The issue is then at the point of the lift where the lower back starts to round.