One of the most important steps in kneading dough is activating the yeast. Contrary to the popular belief that only fresh yeast is live and dry yeast is not, there is no such thing as dead yeast.
As many know, yeast is actually a microorganism, from the fungus species (brewer's yeast), which comes alive under the influence of heat and begin to consume the carbohydrates in flour, sugar, etc. In the process, they release carbon dioxide, CO2 and alcohol.
At lower temperatures, these microorganisms are asleep. They come alive when they are warmed and placed in a suitable food environment. Dry yeast has a much longer shelf life, it's is easier to store and is usually not pre-mixed with warm water and sugar for it to foam up.
But this process is recommended for fresh yeast, because it will make it clear if the microorganisms are alive and if the dough can rise. When mixing yeast with warm water (but not hot) and sugar, you'll be able to see it activate or in other terms, it will foam up.
But when the yeast doesn't activate there are several reasons for it. In first place, it can be assumed that it was not stored properly and the microorganisms in it have died. In this situation, it makes no sense to use it and it is better to just throw it away.
Another reason why yeast does not foam up is because you've either put too little or too much sugar to activate it or you have added salt to it. The inactivation of the yeast may be due to the temperature of the water in which you have dissolved it (too low or too high), as well as the temperature of the room in which the yeast filled container is placed.
