[Footnote 2:  The gerund is the neuter singular of the future
    passive participle used as a noun, and has the same formation. 
    (Cf.  Sec. 374.d.)]

404. The Gerundive.  The gerundive is the name given to the future passive participle (Sec. 374.d) when the participle approaches the meaning of a verbal noun and is translated like a gerund.  It is the adjective corresponding to the gerund.  For example, to translate the plan of waging war, we may use the gerund with its direct object and say consilium gerendi bellum\; or we may use the gerundive and say consilium belli gerendi\, which means, literally, the plan of the war to be waged, but which came to have the same force as the gerund with its object, and was even preferred to it.

405. Compare the following parallel uses of the gerund and gerundive: 

GERUND GERUNDIVE
Gen. Spes faciendi pacem Spes faciendae pacis
Dat.  Locus idoneus pugnando Locus idoneus castris ponendis
A place suitable for A place suitable for
fighting_ pitching camp_
Acc.  Misit equites ad insequendum Misit equites ad insequendos
hostis
He sent horsemen to pursue He sent horsemen to pursue
the enemy
Abl.  Narrando fabulas magister Narrandis fabulis magister
pueris placuit pueris placuit
The teacher pleased the The teacher pleased the
boys by telling stories_ boys by telling stories_

a. We observe

(1) That the gerund is a noun and the gerundive an adjective. (2) That the gerund, being a noun, may stand alone or with an object. (3) That the gerundive, being an adjective, is used only in agreement with a noun.

406. RULE.  Gerund and Gerundive.

  1. The Gerund is a verbal noun and is used only in the genitive,
  dative, accusative, and ablative singular.  The constructions of these
  cases are in general the same as those of other nouns.

2. The Gerundive is a verbal adjective and must be used instead of gerund + object excepting in the genitive and in the ablative without a preposition.  Even in these instances the gerundive construction is more usual.

407. RULE.  Gerund or Gerundive of Purpose. The accusative of the gerund or gerundive with ad\, _or the genitive with causa\[3] (= for the sake of), is used to express purpose._

GERUND                        GERUNDIVE
Ad audiendum venerunt or    Ad urbem videndam venerunt or
Audiendi causa venerunt     Urbis videndae causa venerunt
They came to hear           They came to see the city

    [Footnote 3:  causa\ always _follows_ the genitive.]

NOTE.  These sentences might, of course, be written with the subjunctive of purpose,—­venerunt ut audirent\; venerunt ut urbem viderent\.  In short expressions, however, the gerund and gerundive of purpose are rather more common.