[The context is that Lucretius is praising Epicurus, his authority. I'm proposing a parsing of the hard part. After the parsing, I have a question about the use of the subjunctive in this passage.]
Cuius ego ingressus vestigia dum rationes
persequor ac doceo dictis
Whose tracks I have followed while I pursue and teach his ideas
[ What follows is the hard part. As I understand the idea, it is that every thing created is controlled by a law of nature that forces that created thing to remain just what it is, so long as it may exist, which existence is limited in time. I could not have seen this without the English translation.]
quo quaeque creata
foedere sint, in eo quam sit durare necessum
nec validas valeant aevi rescendere leges.
[Here is my parsing, mostly guesswork.]
quo: "whereby", conjunction
quaeque: "all things", pronoun, neuter, plural, subject of "sint"
creata: "made", adjective, neuter plural, modifies "quaeque"
foedere: "by law", ablative singular
sint: verb, "would exist"
quam: "and how", conjunction
in eo: "by that law", "eo" is a demonstrative pronoun, antecedent is "foedere"
durare: "enduring"; present active infinitive used as a neuter substantive
sint: "would be"
necessum: undeclined adjective, "imperative", predicate adjective, modifies "durare"
[ the remainder is not so hard ]
validas . . .aevi. . . leges: "the strong laws of time"
valeant: "would they be strong enough"
rescinder: "to annul", infinitive complement to "valeant"
Much of this passage seems to be indirect discourse, in that Lucretius expounds upon the doctrines of Epicurus. Do I have this right?