Mishnah Berurah — משנה ברורה

One may rely on the R"i - And it is permitted to wear that garment. And above in se'if 1, where he (ie. the Mechaber) wrote that "whenever possible, it is better to be concerned for the opinion of Rabbeinu Tam", that means when it's possible to easily find another pair of ציצית, but one is not required to [make an effort to] go searching after them if they're not right by him or if searching for another pair would cause him to miss praying with a congregation. And here [in se'if 3] he (the Mechaber) wrote "where it is not possible", meaning in a situation where it's not possible to find ציצית at all, on can rely on [the opinion] of the R"i. Nevertheless, one should not make a blessing on them and it is forbidden to go out on Shabbos with them to a public domain. And similarly, it can be permitted in an extenuating circumstance also when a bit of the tassel (ie. the winded up and knotted section above the loose strings) itself became severed [Responsum of ?? - see there]. And you should know that all of this [that has been discussed] is not referring to when the string tore at the place of [where it's inserted into] the hole, for [if it tore] there, even according to the R"i, it is invalid on a Torah level. And it's obvious that when the ציצית become invalid, one must take them off [of his body]. And it's not enough to just stand in one's place and to not walk four cubits because the law of four cubits (ie. there is a Rabbinic level law that one should not walk four cubits without ציצית on, but standing in place without ציצית would not violate that) is only applicable when you're walking without ציצית [at all] (But here where you're wearing invalid ציצית, which is forbidden to do on a Torah level, it's not good enough to just stand in place because even that is forbidden). One should also not fix the ציצית while the טלית is still on him, rather he should remove it and then fix it.