That human beings live in great numbers, the rivers are very likely to become polluted.

Why can some non-progressive verbs be used in the progressive form?

Under what circumstances do we use singular verb with a collective noun denoting animate beings as subject?

Using an appropriate cohesive device:

I inherited some skills from my mother. Those skills have been put to the rest.

Using discontinuity:

The days when he could boss me around are gone.

Using extraposition:

To drive without a license is illegal.

Using fronting:

He threatened to take his revenge on his father’s murderer, and he will take the revenge.

Using a non-finite clause as subject:

It is difficult to study English well within 2 years.

Combining two sentences using a contrast-adjunct:

We live in poverty. Our neighbours have money to burn.

Combining two sentences using a preference-adjunct:

He did not ask for help. He decided to work it out on his own.

Combining two sentences using a condition-adjunct:

The weather keeps fine. I see no reason why the village fete should not be a success.