The indefinite article is a basic part of English grammar.

He measured the humidity using a psychrometer.

The guard is guarding the gate.

When he gets angry, his veins that bulge when he is angry become visible.

He is inclined to sentimentalism.

This box is filled with plenty of buffering material to protect the delicate glass products.

I read this weekly magazine every week.

His thought is very unique.

He has an illustrated reference book of insects.

The government is building a nationwide surveillance network.