Basic Rules

Variations in the Spelling of Prepositions 

1. The prepositions без, в, из, к, над, от, перед, под, and с may add an -о to their endings. This takes place in front of a cluster of consonants, and with varying frequency – depending on the preposition. The most frequently encountered forms are во, со, and ко:

во власти	in the power
во Франции	in France
со смехом	with laughter
со многими	with many
ко мне	to (towards) me
ко дну	towards the bottom

Other forms, such as изо, надо, and подо occur less frequently (usually, when the first letter of the cluster is "л" or "p"):

изо рта	out of the mouth
надо лбом (or: над лбом)	above the forehead
подо льдом	under the ice

Безо occurs only in front of "весь" and "всякий":

безо всего	without anything
безо всяких разговоров	without any arguments (lit. conversations)

Finally, certain prepositions which govern the instrumental case take a final -о in front of "мной" ("мною") and, optionally, in front of "всем", "всей", and всеми:

надо мной	above (over) me
подо мной	under me
передо мной	in front of me
со мной	with me

 

надо всем	or	над всем	over all the, above everything
подо всей	or	под всей	under all the
передо всеми	or	перед всеми	in front of everyone, ahead of everyone
со всеми	(nо variant)		with everyone, with all the

   (a) In two compound prepositions (of comparatively infrequent usage) an -о is added to а в preceding a vowel: во имя (in the name of), and во избежание (to avoid). These two prepositions govern the genitive.

   (b) О becomes об in front of non-yotated vowels:

об Америке	about America
об уроке	about the lesson

Otherwise it remains as o:

о Японии	about Japan
о его сыне	about his son

О becomes обо in front of "мне", and, optionally, in front of "всей", "всём" and "всех":

Они говорили обо мне.	They spoke about me.
Она обо всём подумала.	She thought about everything.

Isolated Rules 

1. Между seldom governs the genitive case, and only with similar or identical objects:

между деревьев (gen.)

or

между деревьями (instr.)

between the trees

But only the instrumental would be used in:

между деревьями и кустами

between the trees and the bushes

2. Перед and до may express the same idea when referring to time:

перед завтраком

or

до завтрака

before lunch

However, перед implies a shorter period of time:

Надеюсь увидеть вас до экзаменов.	I hope to see you (sometime) before the examinations.
Он позвонил мне перед экзаменом.	Не telephoned me (shortly) before the examination.

3. The usage of пo in a distributive sense deserves comments.

   (a) Один (stated or implied) stands in the dative:

по одному рублю	one ruble apiece
по килограмму сахару	a kilogram of sugar each

Likewise, with the nouns "сотня", "тысяча", "миллион", etc.:

по сотне сигарет	a hundred cigarettes each
по тысяче долларов	a thousand dollars apiece

   (b) All the numerals stand in the nominative:

по три рубля	three rubles apiece
по два ученика за партой	two pupils at each desk

4.  The following prepositions are no longer used as adverbs:              

вместо; вопреки; кроме; посреди; сверх.

The following are used very seldom as adverbs: возле; вроде; около; подле.