[QUOTE=RealityChuck]
The ideas about time travel and what someone does to change history are varied. Some I’ve come across:
The changes don’t go into effect because you’re only changing your own personal timeline (Alfred Bester, “The Men Who Murdered Muhammed”)
The past is a part of collective assumption, so people only see it in current terms (George Alec Effinger, “The Bird of Time Bears Bitter Fruit”)
Time doesn’t like to be changed, so it will do what it can to get things back the way it wants (Fritz Leiber, “Try and Change the Past”)
Any small change can create massive changes over time (the aformentioned Ray Bradbury, “A Sound of Thunder”).
You can’t change major events, but you can change small things (a Twilight Zone episode, “Back There”)
[/QUOTE]
And don’t forget "If two mutually exclusive events could happen, only the one with overwhelming sentient force will be allowed to prevail.
[QUOTE=NoClueBoy]
Eddies in the time stream.
(Oh he is, is he? Who’s Eddie?)
[/QUOTE]
And this is his couch?
(and it’s the space-time continuum) 
Si
[QUOTE=BrainGlutton]
But in “Genesis of the Daleks,” the TimeLords sent the Doctor on a mission for the specific purpose of changing history; suggesting it is known to be possible, but perhaps there is a TimeLord law against doing it without government authorization.
[/QUOTE]
But in the end, The Doctor didn’t change history.
The Doctor can, and does, muck about in history all he wants - but he can’t go back in his personal timeline. Once he lands the TARDIS somewhere, he becomes part of established events and if he goes back, he risks creating a paradox in the timeline in which he exists. See Father’s Day…he knew it was a Very Bad Idea going back again with himself and Rose already there, he tried as hard as possible to avoid running into them, failed, and the whole world was nearly devoured because of it.
As soon as he landed on the SS Madame Pompadour and popped though the fireplace into her life, he became part of that timeline - going there in the TARDIS would mean risking running into himself and effing up the universe. Again.
For some reason though, this doesn’t appear to apply to the Doctor if he has regenerated.
[QUOTE=RealityChuck]
Later, it just was (the 5th Doctor refused to go back to rescue Adric, but just said, he couldn’t and didn’t give a reason).
[/QUOTE]
Well, the reasoning for not rescuing Adric was fairly obvious: He was from the start a badly conceived & poorly written character; With a newly regenerated Doctor and three new companions, the TARDIS was a little crowded (there were too many regulars - the show works best with the Doctor & one companion); Of the three new companions, he was the least likeable; And he was portrayed by a sorta cute but woefully pitiful actor (Matthew Waterhouse) - who was, by several sources, a pain in the ass to work with. (Killing off Adric might not have been done for the shock value of seeing a regular player bite the dust, it may have been, in part, a ‘fuck you’ to Waterhouse on the part of producer John Nathan-Turner.