Pronunciation of конечно

Question

So recently I came across this word and I noticed that it's actually pronounced конешно. I thought I was missing something but the "voiced consonants rule", does not mention the letter ч being pronounced as ш. Does this always happen when ш comes before н or is this word special?

Answer 1

A Comprehensive Russian Grammar by T. Wade gives a rule for the pronunciation of -чн-

The pronunciation of -чн-

(1) -чн- is pronounced [ʃn] in certain words (конéчно [kʌˈnɛʃnə] ‘of course’, нарóчно ‘on purpose’, очéчник ‘spectacle case’, прáчечная ‘laundry’, скýчно ‘boring’, яичница ‘fried eggs’), as well as in the patronymics Ильинична ‘Ilinichna’, Сáввична ‘Savvichna’ and Никитична ‘Nikitichna’.

(2) However, the pronunciation [ʧn] is used in more ‘learned’ words such as áлчный [ˈalʧntj] ‘greedy’, античный ‘ancient’ добáвочный ‘additional’, and конéчный ‘ultimate’.

(3) -чн- is pronounced either as [ʃn] or [ʧn] in бýлочная ‘bakery’ and молóчная ‘dairy’. Коричневый ‘brown’ is pronounced with [ʧn].

Note Ч is also pronounced [ʃ] in что ‘that’ and чтóбы ‘in order to’.

This rule describes the Standard Russain based on Moscow pronunciation. Regional pronunciations may differ. For example, in Saint Petersburg people, especially older generation, would say конечно with [ʧn].

When it comes specifically to конечно in Moscow dialect, a choice between [ʧn] and [ʃn] also depends on a specific meaning:

• конечно as 'surely' is pronounced with [ʃn];
• конечно as 'finite' or 'limited' is pronounced with [ʧn].

Answer 2

Except конечно and что pretty much every word containing -чн/чт- can (and many must) be legitimately pronounced without transition to ш, and while конеЧно and Что being pronounced literally are perceived as acceptable albeit uncommon, pronunciation of поШта, обыШный instead of поЧта, обыЧный, or тоШный, лиШный instead of тоЧный, лиЧный etc. not only militates against normative pronunciation but creates difficulties in communication.

So the rule of thumb would be pronunciation of these phonemes as they're written. While конечно and что can be simply memorized.