Quote Originally Posted by radomir
I will study more calmly your answer. The confusion was due to different origin of я in неделям and музеям. For my use and to make clear if I have understood, I say
Soft indicating vowels represent either
a) softening of preceeding consonant, incorporating a ь belonging to the previous consonant: неделя ->недель+а , недель+ам ->неделям; or
b) two sounds й+V because of
b.1. position: beginning of the word, Я; after any vowel, моя
b.2. adition: of a previous and independent й; музей, музей+ам ->музеям.
Is it OK?
Basically, yes. But b.2. is the same as b). Basically there is a rule that in Russian you can't write Й followed by another vowel. Note this only applies to Russian grammatical endings and native Russian words.