Legal writing is not the most comprehensible in any language. As described in the page Legal writing, "Perhaps most obviously, legalese suffers from being less comprehensible to the general public than plain English, which can be particularly important in both private (e.g., contracts) and public matters ..."
It is always wise to read legal document carefully.
One way that I found effective to deal with legal writing is to parse it mechanically. This works because the main goals of legal writing are accuracy and precision. Here is how I would parse it,
You will not be charged interest (on each new purchase) (for a minimum grace period of 21 days) (if you pay the new balance total in full) (by the total minimum payment due date for the account statement) (on which such new purchase first appears).
Here is how we can interpret the sentence:
- You will not be charged interest on each new purchase.
- You will not be charged so for a minimum grace period of 21 days.
- On the condition that you pay the new balance total in full
- And you must pay so no later than the mentioned due date (the total minimum payment due date as printed on the account statement)
- The mentioned account statement is the one that the new purchase first appears in.
It might still be not quite easy to understand, even after broken down like that, but that's the nature of legal writing. The trickiest part is the term grace period.
The "minimum grace period" part guarantees you that if you don't "fall out of grace", you have at least 21 days to purchase things in advance.
And because when you make a new purchase, its record will appear on the next month statement, the actual number of days you can make a purchase before you need to actually pay your money would be somewhere between 21-52 days (i.e., the grace period plus number of days in a month). In short, you need to pay only once a month. Be sure to pay no later than the due date.
This link might be able to explain the "grace period" in more details: http://www.creditcards.com/credit-card-news/grace-period-avoid-paying-interest.php.