I wonder if some verbs can not be passive voice. I was writing a sentence.
I was thinking that the case doesn't do it itself, So I looked for the passive voice of split off from the internet,but I couldn't find any. So I wonder if some verbs can't be passive? Or it is doesn't matter passive or not.
There is no particular reason why the prepositional verb "split off" could not be used in passive voice. All you need to do is add "be" and use past participle of "split". Since the original sentence has 'split' in past tense, so you use the same form of 'be' ("was"):
The top case was split off from the rest of the parts.
The sentence does not really explain who or what acted on the top case but it can be accepted as implied.
In your example here, split off is used intransitively. Intransivie verbs or verb phrases cannot be passive. "[F]rom the rest of the parts" is an adverbial, not a direct object. So, no, in your case, this sentence can't be passive.
There is no agent here, no actor doing or causing the the splitting off. These are called unaccusative verbs.
"The picture fell off the wall."
In this case (as in yours) nobody is doing or causing the falling. It just happened. The picture and the top case are semantically similar to the "victim" (object, patient) of the verb.
By the way, split off isn't always unaccusative or intransitive, but in this case, it is.
You can also use it with a present perfect tense:
The top case has been split off from the rest of the parts.
