I have one month of Xbox live. If I register my 1 year subscription, will it stack? Never tried, just wondering, don't want to waste my money.
Yes, buying more Xbox live does 'stack' as you thought.
When you have 3 months left of Xbox live and you buy a year more, you now have 1 year and 3 months left.
It is also always consecutive and immediate, you have from now until your subscription runs out and there is no way to 'hold' or 'pause' your subscription for any time that you are not using it.