Structure , pressure and Temperature how do they effect stability?

Let's start with the easiest and work up.

In general, increased temperature has a negative effect on stability. We can see this easily in everyday life: water boils at higher temperatures, and many compounds decompose at higher temperatures. Indeed, this concept is the reason we have refrigerators and freezers to preserve our food for us. Lower temperatures make food last longer.

Conversely, increased pressure generally has a positive effect on stability. This is seen in the higher temperature required to boil pressurized liquids. Take also the example of pressurized gases. While in their high-pressure container (be it a metal tank, or the tires of your car) they are fairly stable; release the pressure, however, and the gases expand at a rapid rate.

Finally, the structure of a molecule will have an enormous effect on its stability. This category is a lot more esoteric; while temperature and pressure are each 1-dimensional, structure has a lot of different elements to it that influence a molecule's stability. In general, the most quantitative measure you can point to is the energy contained within the bonds. The most stable molecules have low-energy bonds, and unstable molecules have high-energy bonds.

For an example, see the following combustion reaction. A fuel, such as propane, has high-energy C-H bonds, and all it takes is a spark to break them to form lower-energy C-O and H-O bonds:

C3H8 + 5O2 --> 3CO2 + 4H2O

Hope this was helpful :)