I'm trying to translate the sentence "Beautiful things exist to be understood." I believe "Res pulchrae intellegendus sunt" is quite close, but I think the sentence would sound better if it actually used the proper verb for "to exist".
What would be a better, more accurate translation?
Review of "Res pulchrae intellegendus sunt"
First, the gerundive, like regular adjective, should match its subject in gender, case and number. In that case res is f. pl. nom., so that yields: "Res pulchrae intellegendae sunt". Second, I think you are right to feel the sound is somewhat not what you are aiming to. The tone of "Res pulchrae intellegendae sunt"is usually rather: "beautiful things should(must) be understood" than "they exist in order to ..."
Suggestion
There might be several ways to render "exists", depends on context. I suggest to use here a passive participle. A possible candidate would be creatae (*). To express purpose, or the "infinitive" of reason we may use several different constructions (also). Here, we are dealing with passive infinitive, so we can chose ut+subj format, we will end up with (the word-order is quite flexible):
Res pulchrae ut intellegantur creatae sunt.
Or, instead creatae, to simply use the the verb exsisto:
Res pulchrae ut intellegantur exsistunt.
(*) One may raise some valid philosophical argument against this selection, but "(in order)to be understood" is, for me at least, makes it good fit).
You can say "beautiful things" simply by using the neuter plural pulchra.
Existence can be expressed by esse, so sunt is fine.
The gerundive must be congruent to the subject, so it has to be intellegenda.
So pulchra intellegenda sunt.
Or you could say pulchra sunt ad intellegendum or pulchra sunt ut intellegantur.
