These are:
why, where, how, when
They are usually placed at the beginning of a question.
Examples:
- Why are you so late?
 - Where is my passport?
 - How are you?
 - How much is that coat?
 - When does the train arrive?
 
Notice that how can be used in four different ways:
1. meaning            'in what way?':
          How did you make this sauce?
          How do you start the car?
2. with            adjectives:
          How tall are you?
          How old is your house?
3. with much            and many:
          How much are these tomatoes?
          How many people are coming            to the party?
4. with other adverbs:
          How            quickly can you read this?
          How            often do you go to London?
