Gerunds in do have the same construction with ablative, and gerunds in dum with accusative cases, as
Scribendi ratio conjuncta cum loquendo est:
The means of writing are joined with speaking.
Some things are written precisely after the writer’s way of speaking. We once, for example, saw the following notice posted in a gentleman’s preserve.
Whear ’as Gins and Engens are Set on Thes Grouns for the Destruction Of Varmint, Any trespussing Will be prossy-Cuted a-cordin Too Law.
Locus ad agendum amplissimus:
A place very honourable to plead in.
It may be questioned whether Cicero would have said this of the Old Bailey.
When necessity is signified, the gerund in dum is used without a preposition, the verb est being added.
Cavendum est ne deprênsus sis: