It's not an idiom, just a natural use of emphasis that is only intuitive to natives. In the present, you'd say "io non decido" to simply state a fact (I don't decide or I'm not deciding), but "non decido io" to mean "it's not me who decides", "I'm not the one making the decision", "it's not up to me to decide": the strong implication is that the action (deciding) is not (cannot be) performed by me, but someone else. You could do the same with other verbs, e.g. "pago io" (I'm the one who's paying - i.e. no one else dare pay): typical sentence when paying a round at a café or a pub. Now transfer the action to the past, and you have either "non ha deciso lui" (it wasn't him who decided, that one time), or "non decideva lui" (it wasn't him who decided, over a period of time, i.e. he wasn't the one making decisions).
To quote a recent example from an Italian newspaper: "Sopra di sé aveva il sindaco e la giunta. Le decisioni non le ha mai prese lui. [...] Contava tantissimo, non crede? - Ma non decideva lui." (above him he had the mayor and the council. He was never the one to make the decisions. [...] He was very important [lit. he counted a lot], don't you [formal] think? - But he wasn't the one who decided). Note that here the interviewee used emphasis twice, first with an Object-Verb-Subject order (doubled by the clitic) and then with Verb-Subject (which is a subcase).
I understand how the sentence emphasises that it was HE who did the not-deciding. But in English there is a definite difference between 'it was not he who decided' (neutral, just factual) and 'it was not up to him to decide' (carries a judgement about whether he should have been deciding or not). Does Italian not make the same distinction?
I wonder if this is also an imperfect/present perfect distinction.
Present perfect: "Non ha deciso lui." = "it was not he who decided"
Imperfect: "Non decideva lui." ~= "It was not he who was deciding" (or, "who was doing the deciding")
In which case, the second one seems like less of a stretch to the translation here, "It was not up to him to decide."
I'm just a learner, though, so someone else please correct me if this seems unreasonable!
When five words are missing that, in English, would change "He didn't decide" to "It was not up to him to decide..." it's an idiom, irrespective of subject pronoun placement, and the specious argument above notwithstanding. Give me "Non era per lui a decidere," or something, perhaps "Non stava a lui decidere." Don't give me three words in Italian that mean eight words in English and tell me it's "just a natural use of emphasis that is only intuitive to natives." There's nothing natural about it. That's what's called "an idiom."
I think there are two separate issues in this sentence which are getting confused.
No 1. Putting the lui, the subject of the verb which would more usually go before the verb, at the end of the sentence. This is what f.formica, moderator, explains at the top of this thread, is "not an idiom, just a natural use of emphasis that is only intuitive to natives." It emphasises that it was HIM we are talking about.
No 2. The translating of a short simple sentence 'non decideva lui' - not as 'it was not he who was deciding' but as 'it was not up to him to decide'. This introduces extra meaning which is not obvious on the surface, i.e. the idea that it was not his responsibility or privilege or business to make the decision. That's why people are asking 'is this an idiom?' . And it raises the interesting question posed by PhillipBur below, does it apply to all verbs in sentences phrased like this?
Unless it's an idiom, I don't think this makes sense either. They don't mean the same thing in English at all. to "be up to him to decide" would imply some measure of responsibility, not merely inaction. As it reads, it seems like only "he didn't decide". That doesn't hint one way or the other whether it was up to him (i.e. his responsibility) to do that. I'd also love it if a native speaker could clarify if this is idiomatic.
Further down in this thread someone seems to explain it as being about syntax, which makes sense. Because "lui" comes last, the emphasis is on him, whereas if lui came first, it would be the simpler "He didn't decide". Hope I've understood that correctly.
OK, but "He was not deciding" needs a context or should be replaced by "He didn't decide" and that only. I don't know yet when you use the imperfect tense in Italian and when you use the perfect tense. It differs in languages, so I am careful about simply transferring the tenses to another language, as it is done here in the course.
I know. It's been a pain so far. You can try reporting it, but I try to make it sound different from other tenses to avoid confusion. And so far, what I've learned is that you use the past imperfect for continuing or habitual actions in the past. Perfect for a one time deal that happened and stopped. That's the best explanation I can give. They have the tips and notes in the category now.
What MadelynWri wrote sounds 'clunky' because it's not correct. We writes & say "used to" for positive statements, but "use to" for negative statements (with not) and with questions (with did or didn't). Didn't you use to be taller? Did you use think that? I used to think so. In speech the d & t sounds run together, so it's not important, but we should be correct when we write.
18SEP'16 I can guess* how you can end up with "It was not up to him to decide." but as there are 49 comments on this sentence and no native speaker has yet supported this translation I have reported to DL that there are these many comments and requested them to please explain their translation so we don't have to guess anymore.
*The subject "lui" is after the verb and therefore stressed. Imperfetto is used when the action was continuous (was not deciding) or repeated (did not use to decide, used not to decide, would not decide). Other uses of imperfetto: https://www.duolingo.com/skill/it/Verbs%3A-Past-Imperfect
So then I end up with: "He, he was not deciding," which could imply(!) it was not up to him to decide (= it was not his task), but could also mean that he just took too long to come to a decision and maybe someone else was more decisive (=he did have the right to decide but failed to do so).
Read f. Formica's response. He is a native speaker. In my lesson the DL question is to translate from English to Italian: It was not up to him to decide. The correct answer given by DL is: Non stava a lui decidere. IMHO I think the correct ENGLISH translation should be: It was not for him to decide.
I don't know what the exact intention of this sentence is to a native Italian speaker, but if it is as suggested, I would use the simple sentence "It' was not for him to decide". Using the preposition 'up' with secondary preposition 'to' really makes it impossible to explain logically, except to say that's how it is.