Consider the following two sentences:
“The cow ate the grass.”
“The grass was eaten by the cow.”
The first sentence is what we call active voice. From the very start, both the subject and the actor of the sentence (the cow) is clear. We see it take action (ate) on the recipient of its action (the grass). By contrast, the second sentence is what we call passive voice. The subject (the grass) is now the recipient of the action (eaten), and we don’t find out who or what did the eating (the cow) until the end.
In modern English usage, we tend to favour active voice where possible. The grammar is simpler, so it’s easier to express and understand. In this case, it uses a past simple verb (ate) to express direct action. The passive version employs a past simple auxiliary verb (was) and the past participle (eaten).
There are several reasons why people don’t like passive structures. Sometimes they think that the passive voice is overly formal or snobbish. The grammar is more complex, which can make it harder to follow, even for native speakers, and this problem is compounded for non-native learners. For example, less fluent speakers sometimes believe that the start of a sentence is always the actor, so they might think that the grass ate the cow, however unlikely that may sound. It’s also easy for learners to miss auxiliary verbs, since English is very particular about these and they don’t exist in many languages.
However, passive voice has its uses, and a fluent English speaker should be able to use it properly in both speech and writing. It is used to:
Passive structures used to be more common in English than they are today. It is interesting to read novels and newspaper reports from the 1800s and early 1900s, which use it fairly often.
Some languages feature passive structures more often than English. For example, Finnish uses passive voice quite a lot. Even if a Finn is very good with English, they need to look out for passive expressions in both speaking and writing.
A lot of business today is done between two or more non-native speakers all communicating in English, and using the passive voice incorrectly runs a high risk of misunderstanding. I suggest that you prefer one voice over the other according to the culture you are interacting with. If it is a formal culture, use more passive voice. If it is a less formal culture, choose active voice.
But whichever dominates, always use a mix of the two. Fluent English speakers move between active and passive voice as appropriate for the context. Become a master of both to impress your readers and your listeners!