The sentence

G. Question-word questions (3): ‘Why?’, ‘How?’
‘Why …?’ as a question-word
1. Why …? asks for a reason and we answer with Because (Not *Why*) or a to-infinitive.
2. We can use What … for? in place of Why?: What did you do that for? (= Why did you do
they?)
3. We use Why don’t/doesn’t …? (Why don’t you?/Why doesn’t she?) to make suggestions.
4. We may use Why? or Why not? + bare infinitive: Why wait for him: Why not wait a bit?
‘How …?’ as a question-word; ‘How much?/How many?’
1. We use How?, basically, to ask about manner:
How did your get on I the exam? – Quite well, I hope!
How combines with much, many, and with adjective and adverbs (How far? etc.).
2. We use How much? + uncountable to ask about quantity: How much bread/milk/time?
We also use How much? to refer to cost: How much does this cost? How much is it?
3. We use How many? + plural noun to ask about number: How many people/books …?
‘How’ + adjective or adverb: ‘How far?’
We can use How + adjective (How long?) in place of What + noun (What length?):
How long is that skirt? What length is that skirt?
‘How long…? (for)?’ and ‘How long ago?’
How long …? (with or without for) asks about duration: How long are you here (for)?
How long ago? + past tense refers to a point of time: How long ago did it happen?
Social uses of ‘How …?’
We ask questions with How …? for:
- introductions: How do you do?, answered by How do you do? (Not e.g. *Fine, thanks*)
- health: How are you? How have you been?, answered by e.g. Fine, thanks.
- personal reactions: How was the film? (= What was the film like?)
- offers and suggestions: How about a drink? (= What about a drink?)
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