The tutorial showed how to use 3ds Max to add root bones to animations, but I have always used Blender (Blender is really easy to use) and was too lazy to download 3ds Max. I read some tutorials online to record how to add root bones.
The first step is to import the fbx file (remember to select the automatic bone coordinate system when importing, this is to make the bone position neater)
After importing, remember to press ctrl+a to select rotation for transformation (if not, the relative orientation will change, and the animation playback direction will be inconsistent when importing into ue4)
The second step is to enter the editing mode and add a root bone with shift+a (the root bone's own coordinate system must be adjusted to be consistent with the first-level bone coordinate system, generally no need to adjust) and then use ctrl+p to make root its parent (usually The first level of a character's skeleton is the hip or pelvis, which are the highest level at the beginning)
The third step is to open the curve editor and enter the posture mode. Press the i key in the transformation-position area to create an xyz keyframe for the root.
The last step is to copy the hip's x and z transformation (horizontal displacement, depending on the situation) to the root, and delete the original hip, leaving only the initial position keyframe.
Then export it
Summary: The last step is actually to use the root to record the horizontal displacement information of the animation, and leave the rotation, vertical displacement and other transformations to the next level hip, so that ue4 can perform displacement according to the root when using the root movement, instead of hip, because hip records other information. Also for T-Pose, you need to add the same root bone to avoid mismatch when importing into Ue4.