1.How to hold the first frame when I delay a footage
copy the layer, in footage controls set start/end to frame 0, then set End Behavior to Hold Last or Loop for however many frames you need.. then slide that layer in front of your delayed footage in the timeline..
2.How can I adjust alpha channel, like discreet key control matte?(shrink,erode,blur......)
operators/keying/matte controls
3.Can I network render on different os(pc&mac)at the same time?
can't help you there.. sorry..
-Matthew