Spline tangent creates an animation curve that is __________the key before and the key after the selected key.

Linear tangent creates an animation curve as a curved line joining two keys

What tool is this in the graph editor?

Allows manipulation of the in and out tangent handles individually so you can edit the curve segment entering or exiting the key without affecting its opposite handle.

Unify Tangents retains the relative position of the tangent handles even after tangents are individually adjusted.

Curves before the first key and after the last key will be flat unless you set the pre and post infinity controls to anything other than constant.

Pre infinity repeats the animation curve as a copy infinitely after the last key of animation

What tool is this?

This tool lets you preserve the angle and length of tangents.