Proper use of passive and active voices? [duplicate]

I was taught that whether to use active voice or passive voice depends on what you want to put the emphasis on. Sometimes it's the subject, other times it's the object.

There is a preference for use of the passive in certain scientific genres, where the identity of the agent is irrelevant. For instance, if you describe an experimental procedure in the passive, you invite the reader to infer that, in your opinion, if they do what you did, then they’ll get the same result. That is, you suppress expression of your personal identity/agency, as it has nothing to do with the outcome.

In interpretative or evaluative sections of articles, where authors express personal opinions about implications, causes, mechanisms, improvements, etc., you find more use of personal pronouns, and less of the passive (or at least, that’s my impression from reading papers in cognitive neuroscience, animal behaviour, and related areas).

It is really a matter of how it sounds. For someone learning English I realise that that is not particularly helpful. It is certainly a good thing to avoid repetition, both of vocabulary and of sentence structure. It is also a good thing not to start a number of successive sentences with the word 'I'. Especially this is the case when writing a letter, such as one of application for a job. One needs to vary one's sentence structures and the use of the passive voice does provide one such opportunity. But there are others. I'm afraid I may not have been of much help, but it is largely a question of practice and experience at writing in English.