someone was saying the amount you get in a trade deps on how much damage youve done to them..
so you should give quite a few crappy cities then take them back to get more tech??
Yes, that is the idea. On levels where the AI is at least on par with you research- or otherwise this is a fairly easy way to keep up in techs. How much tech they are willing to give you depends however quite heavily the time and the map size, for example.
From the Middle Ages onwards I don't think should hold your breath for getting MA techs in peace deals, at all. The best you could probably do would be a discount.
And similarly on large or huge maps you probably need to do more than just take one or two measly towns in order to get a significant amount of AA techs.
also when do cities get destroyed when you take them ?
is this when the pop is 1?
Yes that is right.
But there is another requirement and that is culture. If the city has an amount of culture of 10 or higher (first cultural expansion)
from any single civ it will also not autoraze.
ETA:
A city of size 1 that has no culture will also not autoraze if its population is from the same civ that takes the city. (Say, you retake a city that you previously lost.)
Note however, that if you gift away a city with a population from your civ, then that population will become that of the civ you gift the city to. Ugly sentence. Say, you are Chinese and you gift away a city with Chinese pop to the Americans then the pop of the gifted city will change to American. If you take the city back by military force, the pop will stay American. If now you gift it away to the Indians, then pop will still stay American.