Answer 1. Since all the volcanic ash blocks out the sun's rays and is composed of many toxic chemicals including heavy amounts of anti-oxygen chemicals it will make the atmosphere around the volcano full of chemicals that make it hard for life to exist.
Answer 2. Photosynthesis is the process of converting water, Co^2, and sunlight into glucose and oxygen. Sunlight will be blocked out making it difficult for the plants to photosynthesize. Nearby waters will be contaminated too because of all the ash. The amount of C0^2 will be too overwhelming for most plants thereby killing many of them.
Answer 3. People can bring plants indoors and see that the plants are treated with care and properly. An alternative is to grow newer plants or gardens locally if possible. This will be ill-advised if you see lava coming straight towards your town.
Answer 4. Other events that can block out the sun are storms. If powerful enough they can block out the sun. Since the sun is our prime source of energy, we would seriously take a major hit population wise. Some plants might live if we use flashlights or other light alternatives. Animal's will die off and people eventually. Ends pretty bad and more complex.