One of the students always erases the chalkboard after class

- The chalkboard is always erased after class.

The case is very heavy. I can't carry it myself

- It is such a heavy case that I can't carry it myself.

Practice speaking English every day or you can't imporove your English

- If you practice speaking English every day, you can improve your English.

Noah could not come to class because of his serious illness

- if Noah weren't ill seriously, he could come to class.

We should recycle waste paper to keep the invironment clean

- Waste paper should be recycled to keep the invironment clean.