1. Every piece/article of the furniture in this display is on sale at half-price
OR every piece/article of furniture in this display is on sale for half the (regular/usual) price

2.he is abusing his father's money

3. the duty of the new secretary is to answer the telephone, type letters and bookkeep
OR the duty of the new secretary is answering the telephone, typing letters and bookkeeping

4. The new models are not only less expensive but also more efficient

5. He complied with the requirement that all graduating students in education should write a thesis
OR He Complied with the requirement of writing a thesis by all graduating students in education

6. No sooner had we left the shop than it began to ran

7. The population of Karachi is bigger than any other city in Pakistan