I have difficulty understanding the constructions of this sentence. Please help me.
Quod est, eo decet uti et, quicquid agas, agere pro viribus. (De Senectute IX 27)
1) 'quod est' is an idiomatic expression?
2) What's 'eo' here?
3) What's the subject in this sentence?
And this is a meaningless phrase but I want to know the inflection of ordinal number.
the secret of room no. 17(or the secret of 17th room)
in “Quod est, eo decet uti et, quicquid agas, agere pro viribus” (Cicero,De Senectute, 27) the relative clause “quod est” is nothing but the antecedent of “eo” in the main clause “eo decet uti” which has no subject as there is the impersonal verb “decet”(literally, “it is seemly”).
In short, here’s the literal translation:
”It is seemly (DECET) to use (UTI.Present infinitive of “utor”) it (EO.Antecedent of “quod”. Ablative case depending on the verb “utor”) that (QUOD.Nominative neuter singular.Subject of “est”) there is (EST) and (ET) to do/to act (AGERE)whatever (QUICQUID ) you are doing (AGAS) according (PRO.Preposition that takes the ablative) to [our] strengths (VIRIBUS)”, i.e.:
“We must use what we have and adapt our actions to our strength”
with reference to the previous passage where Cato the Elder, then 84, talks about the strength of youth that fail old age so that old men must do what they can in proportion to their strength".
As for the inflection of the ordinal number 17th, note that ordinal numbers are adjectives whose inflection is of the First and Second Declension like “bonus, bona, bonum”.
Thus 17th is “septimus decimus” (XVII) whose genitive masculine /neuter singular is “septimi decimi”, while the genitive feminine singular is “septimae decimae”.
In short, “septimus decimus” must agree in gender, number, and case with the noun it refers to.
For the Latin Numerals see Allen and Greenough's New Latin Grammar for Schools and Colleges, 132-133 at http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0001%3Apar