I've checked words ending with all (or almost all) possible combinations of C1C2V and C1ьC2V type, where C1 and C2 are different consonants, and V is vowel а, я, о or е. The main conclusions are:
- Most of the combinations don't follow the question author's rule.
- But some of them do, and in most of them C2 is
к, л, н or ц.
- Some combinations with C2 =
б, г, м or р follow the rule, too, but they are much less numerous.
- For the rest values of C2 I didn't find a single word following the rule.
Here are the combinations following the rule:
C2 = к
C1 can be any other consonant. No exceptions found (for some rare combinations, like гка or щька no proofs found, as well, but if such words exist, they should follow the rule, too).
C2 = н
- C1 =
б, в, г, ж, к, м, п, с, т, х, ч, ш. These combinations follow the rule, no exceptions found.
- C1 =
д, з, л. Only words with palatalized C2 follow the rule (обедня - обеден, кузня - кузен, мыльня - мылен, but ехидна - ехидн, тризна - тризн, волна - волн).
- For C1 =
ф, ц and щ no examples found.
C2 = л
This case is very irregular. Some combinations follow the rule, some don't, and the majority of them follows it partly. I think, no rules can be successfully applied here, the best way to deal with it is to learn as many words as possible, so that you just remember the most common ones and develop some intuition for the rest.
C2 = ц
I've found examples only for C1 = в, д, л, н, р, т. All of them follow the rule, except for one word: солнце - солнц. For other values of C1 I didn't find any words, but my intuition says that if there are, most of them should follow the rule, too.
Other combinations, which follow the rule: дьб, ньг, рьг, "др" followed by "о".
Words following the rule while the corresponding combination doesn't follow it in general: розга - розог, тюрьма - тюрем, письмо - писем, ребро - рёбер.
I still don't understand, why the situation is as described above, and not some other way. If anyone has any suggestions, please write them in comments! Thanks!
