the first and second rules are easy to understand, but third rule “when ‘一’ is used along or in a number , it’s tone does`t change ” can be a bit confusing !
I think it’s better to say “when ‘一’ used alone or in a sequence of sth, it’s tone does`t change ”
For example, if you say “一yí个苹果” , “一” should be second tone here, but if you say “第一yī个苹果”, then it should be first tone ! Because “the first apple” is in a sequence of “the first, second, third…” whereas “one apple” is not !
