For those of us with memories of subjects, linking verbs, gerunds, and all the rest, it's hard to imagine going to school and learning grammar in a language other than your mother tongue. But this is the case for many people in the world.
To get an idea of what that would be like, consider the example of the passive. if you went to school as an English speaker and just studied grammar in a language like Lefa, you would not learn about the "passive." Lefa has a construction that is used when you don't want to mention who did an action, but it just uses an impersonal "they": "They found the stolen jewels." So your grammar might talk about "impersonal they" sentences, but not "passive" sentences. Then how would you classify a statement like "The stolen jewels were found."? You wouldn't have a way to describe the three way distinction (active, passive, impersonal "they") that (spoken) English makes. You wouldn't understand when the various constructions were used. And you might not even consider using the passive when translating from the language whose grammar you studied that only uses the active or an impersonal "they".
Even having studied English grammar, the uses of these constructions are not always clear. Why do you think you use the passive or the impersonal "they"? These examples could give you some hints.
Rules are made to be broken.
The senator was shot.
Mistakes were made.
They found the stolen jewels.
Short Answer:
Active – in the normal case.
Passive – to downplay the person doing the action, to ensure continuity of subject.
“They” – orally, often with new items.
Long Answer (for budding linguists!):
Passive
A reason frequently mentioned for using the passive In English is downplaying the actor, either because it's not important: "Rules are made to be broken", "I can't call you. My phone is being repaired." or because it's unknown: "The senator was shot." This can also be to avoid blame: "The lamp was broken", "Mistakes were made." This use of the passive is also common in scientific writing to downplay the specific researchers and focus on the results.
Often grammarians suggest using the active in all other contexts. However, the passive is also natural in maintaining focus on a given person as subject. For example, consider the following series of statements:
John was walking down the street.
He cheerfully greeted the other pedestrians.
Suddenly, he was hit by a passing car.
He fell to the ground, hurt.
In this context, the passive is more natural, maintaining focus on John (try reading the above paragraph with: Suddenly, a passing car hit him).
Impersonal "they"
In English, impersonal "they" is very much discouraged in writing. However, it is commonly used in speaking. Like the passive, it is used to downplay the actor. But then the question becomes: besides the written / oral difference, what is the difference between passive and impersonal "they"?
There are many factors involved, and this distinction is not at all clear. But at least in some cases it seems that "They found the stolen money" is more likely when you haven't already been talking about the money, whereas "The stolen money (or just 'it') was found." is more likely when the money has already been mentioned. This goes along with a tendency in English for new items to be introduced as non-subjects - as objects, or at least after the verb (c.f. "There once was a man who .."). Other factors like parallel sentence structure (keeping a participant in the same "role") also seem to figure into the choice.