The prefix s - and - mostly written as heard. But the rule is: if the word starts with a voiced consonant, the prefix must end with z, if a voiceless consonant - on -S.
The prefix or the prefix is the morpheme that serves to form the words. Prefixes in the Russian language 51 thing, there are both Russian and foreign origin. Russian prefixes are the following: no- (imp -) - ( -), voz- (vos-, renewed-), VZ- (vs-), you-, up-, over -, (is-, izo-), na-, Nai-, under-, over- (nado -), -, bottom- (NIS, NISO-), o-, Ob- (about-), monkey- (lead), ot- (OTO -), under- (Podo-), Pere-, pre-, pre- (before-), pri-, Pro-, raz- (RAS-, razo-), s - ( -), with, through (over-), through-, beyond-, inter-, near-, above-, near-, anti-, and to foreign language prefixes: anti-, arkhi-, de-, des-, dis-, in-, inter-, infra-, counter-, Pro-, pan-, post-, proto-, re-, sub-, super-, TRANS-, ultra-, ex-. These boxes are immutable, it is enough just to memorize their correct spelling.
Complexity in the choice of consoles arise when writing words beginning with a - and o-. The prefix a - is used when the negation of something, for example: asymmetric (i.e. asymmetric). The prefix o - is used to denote the direction of action, excessive action, etc., for example: inspect (i.e., not just to watch but to perform this action more carefully and for a long time).
When writing the prefixes Pro - and PRA is also necessary to consider the meaning of the word. The prefix PRA has the meaning "ancient, old, a distant relative of the original", for example: progenitor. In all other cases you must use the prefix Pro-, for example: to utter, to bite.
All rules have exceptions. It's mostly words of foreign origin. They need to remember or check their spelling in a spelling dictionary.
It is especially difficult to students are modified by the prefix pri - and pre-. The prefix in - has a pretty wide range of values, including the following: "about" (sea of Azov); "addition, the approach of accession" (tie); "incomplete action" (to powder); "action completed" (to admit); "a little bit, not for long" (lie down); "concomitant action" (privatnosti); "action in someone's interest" (to hide). The prefix pre - is used when the word has the meaning "very, rather, too" (old) or "through another way" (to overcome).
B or b
If the word starts in a vowel the letter E, E, u and I, after the prefix should write .. Hard sign should be written, if the prefix ends in a vowel, for example: connector, entrance. In compound words if the prefix is a numeral, also written b, such as: bunk. Hard sign also written after most foreign consoles: injection, adjutant. The soft sign after the prefix is not written.