This is where, I find, that a lot of photographers get confused. Knowing which mode to use and when. But it’s really not that hard. It just takes thinking for a second about what you’re trying to accomplish. That’s all.
Single Focus
AF-S (Nikon) or One Shot (Canon)
When to use this:
Use this mode when you AND your subject are not moving.
Why to use this:
It’s particularly useful if your camera is locked down on a tripod and your subject is still; whether that’s a mountain or a portrait subject posing for you. Utilizing a tripod will help assure that you’re not throwing your subject out of focus. However, if your subject leans forward or backward they’re going to throw off your focus. But this is where stopping down your aperture a bit (i.e. shooting at f/4 rather than f/2.8) helps.
Continuous Focus
AF-C (Nikon) or AI Servo (Canon)
When to use this:
Use this mode when either you, your subject (or both) are moving.
Why to use this:
Now obviously this is the mode you’ll want to use when you’re shooting sports, but it’s also what you want to use if you’re shooting any subject that’s moving and that includes portrait subjects. Ideally, if you’re shooting portraits, you’ll be in the previous mode I discussed. But that’s not always practical. Sometimes either yourself or your subject (or both of you) are moving and this will yield better results.