White garments can retain stains or even turn a little yellow after washing. However, there is a very simple method to get white laundry items sparkling clean and white again that doesn't require much work at all.

Simply place the laundry items in a large container of cold water. Cold water works better than hot water. Add some liquid detergent and a little bleach. Liquid detergent dissolves better in cold water than powder.
Knead the laundry a few times until the detergent is well distributed in the water.
Now it is best to leave the laundry in the water overnight so that the dirt can loosen from the fibres. Ideally, leave for 6 to 10 hours.
The next day, you can simply put the laundry in the washing machine and run a normal wash cycle. If you only have whites in the machine, you can add a little bleach to the detergent compartment. The laundry should now come out really white.