Based on observations, the following seems to be the simplest pattern in which the prepositions, are used. My questions about this pattern consist of two parts:
1. If I used this pattern, how often would you estimate that I would make the right choice of в/во, с/со, к/ко, e.g. 40% of the time, or maybe even 60%?
2. What are the most frequently occurring counterexamples to this pattern? I.e. not all counterexamples, because I imagine there are many, but maybe the first two most frequent ones.
- Add an о to the end of the preposition с в к whenever it precedes a word beginning with two consonants, the first of which is either the consonant itself of its voiced/unvoiced partner (the pairs are в/ф, з/с, г/к).
во вторник, во Франции, со зрением, со словами, ко граду, ко книге.
- Otherwise, don't add an о to the end of the preposition.
в воскресенье, с солнцем, к кому, в гражданина, с французского, к планктону.
- Exception: the preposition precedes some form of я: e.g.
ко мне, со мной.
I ask because Russian learners are often told that во, со, ко are used "to make pronunciation of consonant clusters" easier. However, that simply isn't true in general, because в, с, к are simply unchanged before the vast majority of consonant clusters. So I tried to make up the following rule that is more accurate (even if not entirely accurate) but still fairly simple to understand/remember.
Note: These issues are addressed in these four questions (1)(2)(3)(4) on this website. However, they give (for me, who is just beginning to learn this) way too many counterexamples, and most of which do not seem like they are used often, e.g. in set phrases I have never heard of, or with words I have never heard of, or with very obscure grammatical cases which existed in Proto-Indo-European but which are almost entirely extinct in modern Russian.
I would prefer only one or two counterexamples, and counterexamples such that (1) someone is likely to hear and use them frequently (at least once per week), (2) any native speaker, educated or otherwise, is likely to immediately pick up on the difference.