Two letters have been written by him today.

1. I think you have made the correct decision.

2. W. Oughtred had constructed the first slide-rule by 1630.

3. He has saved the file to the hard drive as a PDF.

4. We have installed new anti-virus software.

5. The specialists will have completed the installation of the new security system by tomorrow's evening.

6. Integrated circuit technology had greatly reduced the size of computers by the beginning of the 1980s.

7. People have used abacus since the earliest days.

8. The new application program has somehow corrupted the system files.