Could you help me understand the following sentences?
Which is correct?
My car is twice as expensive as his. The meaning is very clear.
My car is twice more expensive than his. Although many people say this, the meaning is not clear. Many people take it to mean the same as the first sentence. But some people think it means 'three times as expensive', and logic is with the latter people.
If your car costs 1,000 euros and my car is twice more expense, it sounds like my car costs 3,000 euros (1,000 plus 2,000).
twice what? twice 1,000 euros = 2,000 euros
'twice more' means 1,000 + 2,000
So that is why the first sentence is correct. This is in spite of the fact that many people who should know better say "twice more expensive than" when they mean "twice as expensive as".
Why is the first sentence correct? [ as given in the key answer of my grammar book]
How can I explain it to my students?
Thank you very much in advance.