I'm afraid that's just plain wrong. Look in a grammar book
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As I explained in my previous post, the difference between the active and passive voice is determined by whether or not the subject of the verb is the one carrying out the action. If the subject is the actor, it's the active voice; if the subject is not the actor, it's the passive voice. It really is that simple.
In the sentence "I was sitting", who is the person doing the sitting? It's "I", and "I" is the subject of the verb, therefore the sentence is in the active voice.
For comparison, consider the sentence "I was served by the waiter". Who carried out the action of serving? The waiter. Is the waiter the subject of the sentence? No, the subject is "I", therefore this sentence is in the passive voice.
Believe me, when, like me, you write for technical publishers who are ultra-fussy about this kind of thing you get to know it like the back of your hand.