Need help with passive voice

    Need some help from Germanbros. Could you explain how to use passive voice to me? For example, two sentences, "Die Verkäuferin empfiehlt verschiedene Arten Stoffe" and "Mein Freund lud viele Gäste zu seiner Geburtstagsfeier ein". What happens to the Verkäuferin and Freund in these sentences? Do i just throw them out? Please respond.

the passive voice makes the object the subject and vice versa.
here:
Verschiedene Arten Stoffe wurden von der Verkäuferin empfohlen.
and
Viele Gäste wurden von meinem Freund zu seiner Geburtstagsfeier eingeladen.

In the latter you can see well how the Kasus changes as you switch obj. and subj.

omission is optional but one of the main reason the passive voice is employed.
sometimes you want to omit the actor.
e.g. Viele Gäste wurden eingeladen.

as an afterthought: the passive sounds always weaker than active. If you can choose, go for active.