In the following sentence:
If you [buy / want to buy] a pack of cigarettes from a vending machine in Japan, you have to insert your ID card into it to validate your age.
Which version is better to use here?
Someone told me that I should use want to buy because the age-check happens before the purchase.
Is that really true? I thought inserting the ID card was part of buying.
I would use either "To buy" or "If you want to buy", not "If you buy". As far as the language is concerned, both are equally fine.
Being pedantic, "If you want to buy cigarettes" doesn't mean you have to do anything, you just won't get any cigarettes. But that's very pedantic.
Delete "If you buy/want", capitalise "to", the sentence becomes
To buy a pack of cigarettes from a vending machine in Japan, you have to insert your ID card into it to validate your age.
If you definitely have to address someone (2nd person), it is better to use "If you want to buy".
If you want to buy a pack of cigarettes from a vending machine in Japan, you have to insert your ID card into it to validate your age.
If you have to have "If you buy", you will have to modify the tense in the second clause.
If you buy a pack of cigarettes from a vending machine in Japan, you will have to insert your ID card into it to validate your age.
Someone told me that I should use want to buy.
There are many ways to make the statement, one being, using "want to buy". Two other ways of making the statement were also seen above.
Because the age-check happens before the purchase. Is that really true? I thought inserting the ID card was part of buying.
I will be unable to answer these questions as they are opinion based, I am not knowledgeable, more knowledgeable people will be better able to guide/advise you in proper forum related to "countries and regulations", outside of the boundaries of this forum.
Whether the age check happens before the purchase or as part of the purchase is a red herring.
The "If you buy" option is setting up a hypothetical/future scenario in which the rest of the sentence is predicated on having completed the purchase (eg., "If you buy a pack of cigarettes, [then] you will have some to smoke."; note that this is the same construction as "If you go to the store, get me a pack of cigarettes." or "If you click the 'buy' button, your credit card will be charged.").
The "If you want to buy" option is setting a precondition: in order to buy the cigarettes, there's something you'll need to do (similarly, "If you want to buy cigarettes from a vending machine, you'll need exact change.").
