How to solve the problem of underexposure of black and white film?

The sensitivity of film is fixed, for example, 400 film has only 400 sensitivity.

We can think like this.

Sensitivity 400. What is the aperture? What is the shutter?

If you remember it, you can infer the value that can be changed.

Generally speaking, 400 can cope with bright indoor shooting in rainy days, but if the cooperation between your aperture and shutter is not enough to make the picture light up, it is definitely an underexposed photo.