It's difficult to understand his explanation.

This problem is very difficult.

It's difficult to understand the principles of electromagnetism.

It's difficult to understand the principle of this machine.

It's difficult to understand his explanation.

Electromagnetism is a part of physics that studies the interaction between electricity and magnetism.

He is known as a fundamentalist believer.

His explanation was impossible to understand.

Islamic fundamentalism is perceived by some as demanding strict religious norms.

Natural language understanding is an important field of artificial intelligence.