This book forms the basis for understanding the subject.

This principle is fundamental in philosophy.

This building has fundamental importance in the city.

These books are essential for studying.

This information is essential.

The essentials play an important role in the team.

These fundamental (masculine plural) are responsible for important decisions.

These books are the basics of mathematics.

The basic income is paid to all employees every month.

Basically, you should be prepared for any surprise.