Changing the temperature changes the rate of an enzyme-catalysed reaction. A higher temperature increases the rate of reaction - this is because the substrate particles and enzymes have more energy, so they move about more and are more likely to react.
If the temperature gets too high, the rate of reaction will stop increasing because some of the bonds holding the enzyme together break. This makes the enzyme lose its shape. The active site no longer fits the shape of the substrate any more and therefore won't catalyse the reaction and the reaction stops.
The enzyme is denatured and won't return to its original shape.
Each enzyme has its own optimum temperature - when the reaction goes at its fastest - 37 degrees.
If the pH is too high or too low, it interferes with the bonds holding the enzyme together. It changes the shape of the acitve site and denatures the enzyme.
The optimum pH is pH 7 - neutral, but not always - only most of the time.
For example, pepsin is an enzyme used to break down proteins in the stomach. It works best at pH 2 which means its wel suited to the acidic conditions in the stomach.
The higher the substrate concentration, the faster the reaction - it's more likely the enzymes will meet up and react with a substrate molecule. However, if there are too many substrate molecules and the enzyme cannot cope with them all (all the active sites are full) and adding more makes no difference.