The most common question words in English are the following:

WHO is only used when referring to people. (= I want to know the person)

• Who is the best football player in the world?
• Who are your best friends?
• Who is that strange guy over there?

WHERE is used when referring to a place or location. (= I want to know the place)

• Where is the library?
• Where do you live?
• Where are my shoes?

WHEN is used to refer to a time or an occasion. (= I want to know the time)

• When do the shops open?
• When is his birthday?
• When are we going to finish?

WHY is used to obtain an explanation or a reason. (= I want to know the reason)

Normally the response begins with "Because..."

• Why do we need a nanny?
• Why are they always late?
• Why does he complain all the time?

WHAT is used to refer to specific information. (= I want to know the thing)

• What is your name?
• What is her favourite colour?
• What is the time?

WHICH is used when a choice needs to be made. (= I want to know the thing between alternatives)

• Which drink did you order – the rum or the beer?
• Which day do you prefer for a meeting – today or tomorrow?
• Which is better - this one or that one?

HOW is used to describe the manner that something is done. (= I want to know the way)

• How do you cook paella?
• How does he know the answer?
• How can I learn English quickly?

With HOW there are a number of other expressions that are used in questions:

How much – refers to a quantity or a price (uncountable nouns)

• How much time do you have to finish the test?
• How much is the jacket on display in the window?
• How much money will I need?

How many – refers to a quantity (countable nouns)

• How many days are there in April?
• How many people live in this city?
• How many brothers and sister do you have?

How often – refers to frequency

• How often do you visit your grandmother?
• How often does she study?
• How often are you sick?

How far – refers to distance

• How far is the university from your house?
• How far is the bus stop from here?