Learn German: Geh rüber zum Ventil!

rüber is the everyday, spoken-German shortcut for both hinüber (over there, away from the speaker) and herüber (over here, towards the speaker). In daily life, Germans rarely use the full forms and just say rüber regardless of the exact direction.

Does rüber act as an adverb or a separable prefix here?

It can be thought of as both! Here it functions as a directional adverb, but it is also used as a separable prefix in the verb rübergehen (to go over). Because this sentence is a command (Geh rüber!), the verb comes first and the prefix or adverb goes to the end, making it look identical either way.

Why does the sentence use zum instead of just zu Ventil?

In German, you usually need the definite article when giving directions to a specific object. The preposition zu always forces the dative case. Because Ventil is a neuter noun (das Ventil), its dative article is dem. The words zu and dem merge into the mandatory contraction zum.

Why is the verb just Geh?