It's difficult to understand his explanation.

They are my best friends.

He increased the time spent together to deepen his relationship with her.

He goes to the gym frequently.

He goes to the gym frequently.

When he opened the window, fresh air flowed into the room.

When he opened the window, fresh air flowed into the room.

The door opens.

It's difficult to understand his explanation.

We discussed to deepen our mutual understanding.