Mishnah Berurah — משנה ברורה

If he was not careful - This is a difficulty (ie. a question) - For behold, it's a certainty [that the ציצית would be invalid when three ends were severed] if he was careful [to make sure the same four ends were on one side at the time of tying the ציצית, since that means that three of the main strings are now too short], like the Rem"a wrote in his gloss after [the words of the Mechaber]. And we can answer that he (the Mechaber) chose [the words] "[if] he was not careful" because of the end [portion of this se'if]; for he ends [with the words] "but if only two of the ends were severed, it is valid as long as it has [left over enough] to make a loop [around the other strings]" - and that is specifically without being careful. But if he was careful, [then] if two ends were severed from the same side, it is valid, even if there isn't left over enough to make a loop [around the other strings], for there is left over on each one of those, the [entire] other end. But if two ends were severed from each side, and there is not left on each one enough to make a loop [around the other strings], it is invalid, because maybe it was from one string.