Anne Mahoney, Overview of Latin Syntax

Ablative

• “Continetur Garumna flumine, Oceano, finibus Belgarum.” Caesar, BG 1.1

• “Cui dono lepidum novum libellum,
  arida modo pumice expolitum?
  

  ”

  Catullus 1.1-2

The ablative case can indicate a place where something happens. Usually the preposition “in” is used, but not invariably.AG 426, 429

• “Helvetii ipsi in eorum finibus bellum gerunt” Caesar, BG 1.1
• “Ariovistus exercitum castris continuit.” Caesar, BG 1.48

• “Te Campo quaesivimus minore,
  te in Circo, te in omnibus libellis,
  te in templo summi Iovis sacrato.
  

  ”

  Catullus 55.3-5

The ablative case can indicate separation. This is the most basic use of this case, as indicated by its name (“ab + latus” from “aufero”). Usually this construction includes one of the prepositions “ab”, “de”, or “ex”. AG 400-402

• “Gallos ab Aquitanis Garumna flumen, a Belgis Matronea et Sequana dividit.” Caesar, BG 1.1
• “Nunc eum volo de tuo ponte mittere pronum.” Catullus 17.23
• “Nec saevi dolores exciderant animo.” Virgil, Aen. 1.25-26

The ablative case can indicate manner. AG 412

• “Flumen est Arar incredibili lenitate ” Caesar, BG 1.12
• “...sermones varii, in quibus: quo modo se haberet” Catullus 10.6-7

The ablative case can specify something in respect to which something happens; this is the "ablative of respect" or "ablative of specification." AG 418

• “Hi omnes lingua, institutis, legibus inter se differunt.” Caesar, BG 1.1
• “Tu animo offirmas.” Catullus 76.11

The ablative case can denote the time when something happens. AG 423

• “Die constituta Orgetorix ad iudicium omnem suam familiam coegit.” Caesar, BG 1.4
• “Hesterno, Licini, die otiosi lusimus.” Catullus 50.1

Many prepositions take the ablative. AG 220-221

• “Helvetii fere cotidianis proeliis cum Germanis contendunt.” Caesar, BG 1.1

• “Nulla fides ullo fuit umquam foedere tanta
  quanta in amore tuo ex parte reperta mea est.
  

  ”

  Catullus 87.3-4

The ablative absolute is a way of commenting on something that is logically, but not grammatically, related to the main sentence. AG 419-420

• “Eo opere perfecto, praesidia disponit.” Caesar, BG 1.8
• “Lesbia me praesente viro mala plurima dicit.” Catullus 83.1

• “Musa, mihi causas memora: quo numine laeso
  quidve dolens regina deum tot volvere casus
  insignem pietate virum tot adire labores
  impulerit?
  

  ”

  Virgil, Aen. 1.8-11

Ablative absolute phrases almost always include a participle; the most common exception is the idiom "in the consulate of...".

• “Is, M. Messala et M. Pisone consulibus, coniurationem fecit” Caesar, BG 1.2

Some verbs take an ablative, really an ablative of means. AG 410

• “Persuadent Rauracis uti eodem consilio.” Caesar, BG 1.5
• “Quo usque tandem abutere, Catilina, patientia nostra?” Cicero, Catil. 1.1
• “Auro opus fuit.” Cicero, Cael. 31 The idiom “opus est” takes this ablative as well.