What is the difference between the simple past and the passive voice?

Aside from their names sounding alike, there's no similarity at all between past and passive.

Almost every verb in English has several forms, two tensed and two untensed:

1. a present tense form (used for infinitive): go, see, learn, say, fold
2. a past tense form: went, saw, learned, said, folded
3. a present participle form (used for gerund): going, seeing, learning, saying, folding
4. a past participle form: gone, seen, learned, said, folded

The present tense is no. 1. The past tense is no. 2. Only. That's all the tenses there are. Every main clause in every English sentence has to have a tense form (either present or past) as the first auxiliary verb (if there is one), or otherwise as the main verb. And that's all there is for the past tense.

Passive, on the other hand, is not a verb form, and doesn't have anything to do with time, present or past. Passive is a Construction, which means it's not just one thing, but a whole bunch of things done together, like driving a car involves more than just turning the wheel.

Passive is something that happens only to transitive clauses, which means the verb has a direct object. So not every sentence can have a passive form; it has to be the right kind of verb and there has to be a direct object. You don't need a specific subject -- indeed, if you are using passive, you probly don't care or even know about the subject. That's the point of the passive construction -- it focusses on the direct object by promoting it to subject in a new sentence:

• (Someone) has washed the floor --> The floor has been washed.
• (Someone) gave beer to the guests --> Beer was given to the guests.
• (Someone) gave the guests beer --> The guests were given beer.

In each passive example, the main verb is a type 4 form, immediately preceded by a form of auxiliary be. This auxiliary takes whatever form the main verb of the original transitive clause had: present perfect in the first, and simple past in the last two.

And the nouns move around, too. The object becomes subject, the old subject usually disappears (but it can hang around in a by-phrase if you want). So there's a lot going on, which is the norm with constructions. But not with tenses. Tenses are really simple.