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Why is it

обо мне

обо всём

but

передо мной

перед всем

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Philip Seyfi
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    I am not a native speaker, but as far as I know, о(бо) must be followed by the prepositive case, whereas перед(о) must be followed by the instrumental case. Hence the different forms of the pronouns in your examples. – Giorgio Aug 14 '12 at 22:00
  • @Giorgio, I'm not the author of the question, but I guess it about -o ending. – shabunc Aug 15 '12 at 05:40
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    @shabunc: Ok. Then, I can only try to guess. AFAIK the -o is often inserted after a preposition to avoid a consonant cluster. My guess is that the Russians avoid a cluster of three consonants, at least two of which have a similar articulation. Therefore: -б мн- (б м), -б вс- (б в), -д мн- (д н) are avoided by inserting an -o. On the other hand, -д вс- is acceptable (all three consonants are articulated differently), and no -o is inserted. Again, this is only a guess and I am curious to read a more authoritative answer. – Giorgio Aug 15 '12 at 09:20
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    @Giorgio, I think so as well. I think also, that one can find reduced vowel between в and с in всем and this is the reason for 'перед всем' to be acceptable. However, I'm not devoted enough to dig into the matter. – permeakra Aug 16 '12 at 07:29

2 Answers2

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From the answer on Грамота.ру

Передо — предлог, употребляется перед формами мной, мною, а также перед формами творительного падежа существительных, в которых представлены начальные сочетания «ль, л, м, р + согласная».

So передо is used instead of перед before nouns in instrumental case which start with “ль, л, м, р + consonant”.

The rules for о/об/обо are different.
I think it is incorrect to compare two different prepositions which require different cases.

In general, -о is added when it's hard to pronounce the consonant cluster.

Alexey Ivanov
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I'm not sure if some particular rule applies here, but I think that vowel о is inserted before two consecutive sonorants (лнмр) in next word so it's easier to pronounce, мн is the most frequent one.

That I'm talking about is that перед мной is pretty legitimate form (esp. in form пред мной for example), however о is usually added for ease of prononciation. While there is no need to insert о in обо since it's already there.

Petr Abdulin
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  • I'm not asking about why об changes to обо... I'm asking why об does change, but перед does NOT change when used with всё, yet both change when used with я. – Philip Seyfi Aug 17 '12 at 08:11
  • That's exactly that I'm talking about. – Petr Abdulin Aug 17 '12 at 08:21
  • What do you mean by "о in обо since it's already there"? обо is об + о, just as передо is перед + о. The two should follow the same rule, that о is added before consonant clusters. – Philip Seyfi Aug 17 '12 at 10:15
  • No, об is o + б, while обо is о + б + о and is synonym to о. Thus обо мне is actully equv. to о мне. Об is used insted of о if next word is starts from vowel, e.g. об этом. – Petr Abdulin Aug 17 '12 at 10:20
  • I'm sorry, that's what I meant to write... The fact remains that передо IS перед + о, the same way as об is о + б + о – Philip Seyfi Aug 17 '12 at 13:02
  • So what? Anyway, see Alexey Ivanov's answer, since he explains absolutely the same thing, but is more succesful in it. – Petr Abdulin Aug 17 '12 at 13:32