EVF Focus

Endurance swimmers need to develop a long stroke, which means

The catch used by endurance swimmers (and many sprinters) is called the Early Vertical Forearm Catch, or the EVF, for short.

Release the bubbles. Do not begin the catch too early. There is a pause as your arm reaches full extension. If there are still bubbles trapped under your hand when you begin your stroke, your paddle (your hand, wrist and forearm) will slip in the water.
Pop and point. To begin the catch, pop your elbow towards the surface and point your fingertips at the bottom, keeping your wrist straight. This should get your hand under your elbow, so that your hand-wrist-elbow paddle is vertical in the water. This all happens before you begin pulling on that arm.
Catch and drive. This is the "pull". It's a timing thing. The instant your stroking arm catches, the recovering arm drives forward into the water. You are using both arms to move your body over a spot in the water. By acting as an anchor on one side of your body, your catching arm transfers the energy from the forward-moving arm into propulsion for your body. If you have not anchored your stroking hand and forearm when the recovering arm enters the water, that energy is lost - your recovering arm is just poking the water.

No bubbles during the catch or pull. (Bubbles are for sprinters!)
Fingers should not be pointing under your body or to the outside (they should point forward during extension, and down at the catch).
Do not pull the elbow back ahead of the hand. When this happens, the blade of your paddle (your hand and forearm) is tilted. Make sure your hand is under your forearm before beginning your pull.
Sore elbow. The most likely cause is that you are pulling too early and focused on your hand instead of your hand/forearm paddle.
Sore shoulder. This can mean that you are pulling too early, or that you are not rotating onto the other side as you pull.