The subject case is used when the noun is the subject of the sentence.

The subject cases are important for understanding sentence structure.

I created a new sub-page for the website.

The soldiers carried Thompson submachine guns into battle.

The government used moral suasion to encourage citizens to donate to the charity.

The performance of the team was sub-optimal.

There are many subliminal messages in the movie.

Submarine rivers are fascinating natural phenomena.

He was covered in blood after the rugby match.

The submarine fan is an important geological feature in the ocean.