Eos impeditos et inopinantes aggressus magnam partem eorum concidit; reliqui sese fugae mandarunt atque in proximas silvas abdiderunt.

...quartam vero partem citra flumen Ararim reliquam esse, de tertia vigilia cum legionibus tribus e castris profectus ad eam partem pervenit, quae nondum flumen transierat.

Flumen est Arar, quod per fines Aeduorum et Sequanorum in Rhodanum influit, incredibili lenitate ita ut oculis in utram patrem fluat iudicari non possit. Id Helvetii ratibus ac lintribus iunctis transibant.

Ask yourself what kind of clause is in red? What kind of clause is in blue? Why is the organge word infinitive, what is its voice, how do you translate it? What kind of participle is the word in purple, how do you translate it?

The Soane is a river, which flows through the boundaries of the Aedui and the Sequani into the Rhone, of such incredible slownes that it is not possible to determine with the eyes in which direction it flows. The Helvetians cross it with rafts and canoes having been joined.

Ubi per exploratores Caesar certior factus est tres iam partes copiarum Helvetios id flumen traduxisse,

When translated together certior factus est creates a form of indirect discourse: what type of indirect discourse is it and what should you expect to follow?