4. Define the tense of the predicate and put the sentences into the interrogative and negative forms:
1. I am studying biology. 2. He has solved this difficult problem. 3. He is a good biologist. 4. My teacher developed a new plant. 5. These scientists work at a very interesting problem. 6. They began to investigate this problem last year. 7. Animals and plants live under different conditions. 8. Life exists in many places on the earth. 9. Some animals can exist under the immense pressure of the deep seas. 10. Biologists have solved many mysteries of the body. 11. Students of the biological faculty study different subjects.