"But I unfortunately was born at the wrong end of time, and I have to live backwards from in front, while surrounded by a lot of people living forwards from behind. Some people call it having second sight." (S.3.69)
White comes up with a rather ingenious way to explain Merlyn's ability to see the future: he lives backwards through time. Pretty smart! This also explains why Merlyn sometimes gets things wrong or forgets to tell Arthur important info. It's just downright confusing living in a timestream like this.
"Have I told you this before?"
"No, we only met about half an hour ago."
"So little time to pass?" said Merlyn, and a big tear ran down to the end of his nose. (S.3.73)
This is one of the early indications that things don't end up well for Arthur. Merlyn's crying for some reason here... and we should probably trust him. After all, he's lived through this before.
"The time is not yet ripe for you to be a hawk." (S.8.25).
Merlyn puts the kibosh on Wart getting to turn into a hawk at this point in the story. He tells him he'll have to "trust to [his] superior backsight." Again, Merlyn has lived through all of this before, so he knows when it's time for Wart to learn certain lessons.
"This is an anachronism," he said severely. "That is what it is, a beastly anachronism." (S.9.80).
Anachronism (things being out of synch with their proper times) is a big point in White's text. Merlyn explicitly brings up this concept here, when his magical powers summon an 1890's sailor hat instead of a hat more suited to the medieval period.
There was a noise like a railway train letting off its whistle, and, answering to it—riding on it like the voice of the Arabian Bird—Robin Wood's horn of silver began to blow. (S.12.3).
Of course, only someone not confined to the Middle Ages would be able to describe a sound in terms of a "railway train letting off its whistle." The narrator is apparently privy to other time periods, and introduces anachronisms like this frequently.
"We have had a good time while we were young, but it is in the nature of Time to fly." (S.22.90)
Merlyn's getting ready to leave the Forest Sauvage after he has properly tutored Wart. Throughout the book, Merlyn has been sad about various things relating to Arthur, and here he again mourns how quickly time passes by. This is a poignant moment.
"Uther," [Merlyn] said at length, "your lamented father, is an aggressor. So were his predecessors the Saxons, who drove the Old Ones away. But if we go on living backward like that, we shall never come to the end of it. The Old Ones themselves were aggressors, against the earlier race of the copper hatchets, and even the hatchet fellows were aggressors, against some earlier crew of esquimaux who lived on shells. You simply go on and on, until you get to Cain and Abel." (Q.3.50)
It's not surprising that Merlyn has important things to say about living in the past. He has literally lived in the future, and here he talks about a figurative sense of living backwards in time—dwelling on things that happened in the past but that cannot be changed. At some point, the cycle has to be broken, and we have to live in the present and forget about past wrongs.
"Very interesting," he said in a trembling voice. "Very interesting. There was just such a man when I was young—an Austrian who invented a new way of life and convinced himself that he was the chap to make it work. He tried to impose his reformation by the sword, and plunged the civilized world into misery and chaos." (Q.8.25)
This is, of course, a reference to Hitler. Because he has lived backwards through time, Merlyn can bring lessons from the future (his past) into Arthur's world. And here he's barely able to contain his anger at Kay's proposition to just impose your way of life on others if your way is better than theirs and they just won't listen.
"There is a thing about Time and Space which the philosopher Einstein is going to find out. Some people call it Destiny." (Q.10.37).
In other words, space and time are wrapped up together to possibly create destiny, or a fate that cannot be avoided.
"I don't think it is any good complaining about what happened in the past [...] We can't expect other people to side with us when everything is complicated, and happened so long ago." (C.1.16)
Even though he's a straight-up villain, Agravaine shows canny perception here: you shouldn't complain too much about the past, because nothing can really be changed about it. Too bad he couldn't listen to his own advice, hmmmmm?