Epenthetic л: why there is no л in the conjugation of the verb звать between the в and the ё in зовёшь, зовёт, зовём and зовёте?

Question

I understand that an epenthetic л should always appear between any of the five labial consonants б, п, м, ф and в and the sound /j/ (except in front of the sound /ja/), so that, for example, in the verb любить (to love), the epenthetic л appears in the first person singular (люблю́), but not the third person plural (лю́бят – because of the sound я).

Given this, my question is threefold:

1. Why does the epenthetic л not apply in front of the sound /ja/, but does appear in front of any other sound starting with /j/. What is the phonological, orthographical or historical reason for the exclusion of the /ja/ sound ?

2. Irrespective of 1), why is there no epenthetic л in the conjugation of the verb звать after the в and before the ё seeing that this letter starts with the sound /j/? (specifically, why is there no л after the в in зовёшь, зовёт, зовём and зовёте) ?

3. Irrespective of 1) and/or 2), why is there no л in the conjugation of the verb вызвать (to call, to send – вы́зовешь, вы́зовет, вы́зовем and вы́зовете) after the в and in front of the sound /je/, whereas the same sound triggers the epenthetic л in verbs such as трепать (to wag) thoughout its entire conjugation (треплю́, тре́плешь, тре́плет, тре́плем, тре́плете, тре́плют), including in its imperative forms with the sound /i/, which should not per se trigger the epenthetic л (трепли́, трепли́те) ?

Answer 1

Why does the epenthetic л not apply in front of the sound /ja/, but does appear in front of any other sound starting with /j/. What is the phonological, orthographical or historical reason for the exclusion of the /ja/ sound?

The epenthetic l appeared not long before the split of Proto-Slavic because of the law of rising sonority, and behaved differently in different dialects and in different word positions.

Most Slavic languages retained it in the words where it appears in the first syllable. The cognates of плевать < *pjъvati, блюсти < *bjaustei, блюдо < *biutta etc. have the l in the first syllable in all Slavic branches.

Outside the first syllable, the epenthetic l does appear in some West Slavic toponyms, but in recorded West Slavic languages it's almost non-existing.

In South Slavic languages (including Old Church Slavonic) it fell out of use not long after the X century, and before ji, je even earlier than that: корабль < karabion but кораби

East Slavic languages mostly retained it in all positions.

The rule "l before the sound j" means "the original Proto-Slaviс etymons which did originally have the sound j" and not their reflexes in modern Slavic languages, which might not have the sound j at all.

Further development of Russian tends to unify infinitive and finite forms of these verbs.

The epenthetic l has been lost in third plural forms of most verbs: купят, любят, and there is a tendency to change conjugation patterns of verbs that have it.

Хромает has completely supplanted now archaic хромлет, капает prevails over каплет, and forms like сыпет, щипет, трепет etc., that are currently considered colloquial, are on their way to replace the literary сыплет, щиплет, треплет.

Why is there no epenthetic л in the conjugation of the verb звать after the в and before the ё seeing that this letter starts with the sound /j/? (specifically, why is there no л after the в in зовёшь, зовёт, зовём and зовёте) ?

There is no j in this word and has never been. ё is a very recent (by historical standards) Russian innovation. The original Proto-Slavic word had e not preceded by j in its place, and most of its cognates on other Slavic languages still do.

Answer 2

I will answer your question rather not as a professional, but as a narrow man (or dilettante).

1. Because there are no words in russian language like любю. In the same time, there is a word like любя. There is no лупю. But, there is луплю (it is a procces of beating someone or something). I can’t remember a single word in Russian with бю, пю, мю, фю, вю, except one - дежавю (deja vu), but it's a loan word (taken from french language). There is no verb дежавить officialy. Although the Russian language is very flexible, so if I tell a person - "дежавить - это так приятно", then he/she can understand me. For example, it can mean "испытывать чувство дежавю - это так приятно" ("to feel deja vu is so enjoyable"). The verb любить (to love) is derived from любовь (love). The verb лупить is not derived from any noun.
2. Let's look at the words зовёшь, зовёт, зовём and зовёте. Why is there no epenthetic л? Why should it be there? It's not all about letter ё starts with the sound /j/ , but rather it is about form of the word. A form of the word люблю is not the same as a form of the word зовёшь. The word зову has the same form, but not зовешь. The word любишь has the same form as a form of the word зовешь. And you don't see here epenthetic л. Let's find the same forms of words by analogy. зовёт - любит, зовём - любим, зовёте - любите. There are no epenthetic л.
3. And again вы́зовешь is not the same form as a form of the word треплю́. But now, finally, we can see the same form: вы́зовешь - тре́плешь. And it looks like an exception. Yes, the Russian language is full of exceptions :). But you CAN say тре́пешь, and some person easily understand what do u mean. This word can be marked as colloquial (conversational). It is not recommended to use it in literary speech.

An answer on the question of the topic is "because" :). We have many rules in our language, that we take for granted. Becase there are no words: зовлёшь, зовлёт and зовлёте.

Sorry for my english. I think I has some problems with 'a' and 'the', because we don't have in our language similar pretexts (except words 'некий', 'тот'/'этот' and so on).