Yes
The sub-panel need to be fed from the main panel, by way of a circuit breaker connected to one of the breaker locations. Or if your sub-panel has a main breaker installed you can feed from the main panel with a sub-feed lug kit. This looks like a breaker, but is only a point where you can branch off the sub panel.
no
This type of question usually means you aren't ready to do this yourself. Study some electrical material and the National Electrical Code and work this answer out for yourself. If I were to give you an answer, you might attempt to do something you shouldn't be doing, and that may cost someone a shock, a home fire, or their life.
You need to be more specific. Is this your feed?? Is this a home run (the wire that is feeding a circuit). Anyway if this is your feed wire (the wire supplying power to your sub panel) then you will land your two hots , then you will land your ground wire on the lug that is bonded to the panel. Sometimes this is a ground bar that can be screwed to the panel itself and then your neutral will go to the bar that is not touching anything. So your neutral coming in will be tightened down to the same bar as the neutrals that are feeding the circuits. This is assuming you'r are feeding your sub panel with 2 hots, a ground and a neutral.
check your local codes for this
If the sub panel is in the same building remove the bonding screw that bonds the neutral bar to the panels enclosure. Remove the ground from the second ground rod. The sub panel is grounded by the ground wire from the sub panel's feeder. If the panel is separate from the main building treat the sub panel as a separate service. It will need its own ground rods and ground wire from the rods to the neutral bar of the sub panel. Leave the bonding screw in. There will be no ground wire in the conduit between the two services.
you propaly can't sub panel from 100 amp panel. Just not enough amperage to be worth while. i had to up grade t a 200 amp main first and then I was able to take a 60 sub panel from that.
A 100-A sub-panel would be fed from a 100-A breaker.
Not exactly sure what your question is and you need to make sure you are in compliance with electrical code for your jurisdiction. But, generally speaking you install a large breaker (let's say 100A 2 pole breaker to power the sub panel) just as you would install any other breaker in the presently used (hot) panel, and the proper sized cable then goes from old to new and terminates on the main lugs of the new panel. Ground and neutral are usually isolated from each other in the sub panel but depends on other factors as to location of the sub panel, etc.
as long as there is ambient clearance in front of the panel...
Yes it can but it is a complicated job in as much as you have to know wire sizes, ampacity of wires and proper electrical workmanship. Such a project should be left to a licensed electrical contractor to take out the proper permits and call for proper inspections. By taking this route it will leave you confident that if any mishaps happen to the installation down the road your insurance company will be behind you 100%.