You animate the objects of a hierarchy in much the same way you animate anything else. Turn on the Animate button and apply transformations and modifiers at different frames. However, you need to be aware of a few special issues for animating hierarchies.
A child is not constrained by its link to a parent. You can move, rotate, and scale children independent of their parents.
Only transforms are passed from parent to child. Animating a parent object using move, rotate, or scale animates the parent and the subtree attached to the parent.
Animating a parent's modifiers or creation parameters has no effect on its descendants.
Another method for animating hierarchical structures is through the use of inverse kinematics. Using this method you animate child objects and pass the effects up the hierarchy to the child's ancestors. See Introduction to Inverse Kinematics.
When you link an object to another, the link relationship between the child and its parent is determined on the frame when the link is made.
Imagine linking a sphere to an animated box. The figure shows a stationary sphere near a moving box.
At frame 0 the box is beside the sphere.
At frame 50 the box is 20 units away.
Linking the sphere to the box causes the sphere to move with the box. The distance between the sphere and the box depends on the frame when the link is made. Linking the sphere on different frames has the following effects:
Link on frame 0, and the sphere stays next to the box as it moves.
Link on frame 50, and the sphere stays a distance 20 units away from the box as it moves.
When you unlink a child, its frame 0 transforms are taken from the transforms of its parent at the frame when the link is removed.
Imagine a sphere linked to a box moving around the face of a clock. The box starts at 12 o'clock and travels all the way around the face over 100 frames. The figure shows a box moving in a circle with a sphere linked above it.
If you unlink the sphere, it stops following the box. The position of the sphere depends on the frame when the link is removed. Unlinking the sphere on different frames has the following effects:
Unlink on frame 0, and the sphere stops at 12 o'clock.
Unlink on frame 25, and the sphere stops at 3 o'clock.
Unlink on frame 75, and the sphere stops at 9 o'clock.
Links are permanent, regardless of the state of the Animate button. Using a Link constraint to simulate animated links is discussed in Animating Links.