We have special rules for prepostions like these:
1. In the explanatory dictionaries, the semantic difference of prepositions is not indicated in any way.
2. The expression "go to the doctor" - "идти ко врачу" is simply inconvenient for pronunciation.
3. The preposition K is more common, and we use KO in special cases, when there is a meeting of consonants, inconvenient for pronunciation.
On the other hand,
the regularity of phonetic inconsistencies is difficult to determine, for example: to me - ко мне, to everything - ко всему, but: to the meeting - к встрече
В, ВО - If the word after the preposition begins with two consonant letters, the first of which is B or Ф, then in the speech and in the letter the preposition ВО is used:
In a fragment - во фрагменте, in a phrase - во фразе, in France - во Франции, in French - во французском, in a vial - во флаконе
Во фрагменте этого фильма есть очень интересная цитата
- There is a very interesting quote in the film.
Во французском ресторане мы попробовали новое летнее меню
- We tasted a new summer menu in the French restaurant.
In the second - Во втором, on Tuesday - во вторник, in yesterday's - во вчерашнем, in a look - во взгляде, in possession - во владении, in any - во всяком
!! Also, the preposition ВО is used with the following words:
In many ways - Во многом, in many - во многих, in all - во весь, in the name - во имя
С, СО) The principle of use is the same as in the previous pair of prepositions. The letter O is added to the preposition C, if the preposition is used with a word that starts with two consonant letters.
Со стула - from the chair, со мной - with me, со слезами - with tears, со всеми - with everyone, со многими - with many
In the case of the preposition O, the letter Б is added when the word after preposition begins from the vowels А, И, О, У, Э.