Originally Posted by
CoalHillJanitor:
“So when the Doctor used some regeneration energy to heal River in The Angels Take Manhattan, why did he even have any left? 
And apparently River didn't know about the War Doctor, since she reacted as if he had one and only one full regeneration remaining?”
1) Well I suppose after the last regeneration you are not completely drained of regeneration energy
Think about it as a bottle of water you can fill 12 cups up by the end you haven't got enough to fill another but do have some left at bottom of the bottle
2) Well we don't know if River knew or not if The Doctor would tell anybody it would have been her.
I think she just wanted him to save the bit he had left maybe he helped slow the ageing process down so doing that cost him about 100 years of life or he could use it to heal minor injuries on himself (say he got shot he could use it to heal the wound before it killed him)
