No, you cannot say "is lag é", you would use the other verb for to be "Tá mé lag." "He is my boyfriend." is "Tá sé mo chara bhuachaill." It looks as though the minute you put adjectives in, you must use "Tá" and not "Is".
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Can we also say "Is é buachaill," since both "é" and "buachaill" are subject and object because of an chopail is?
Also, how do we know that, in the pronunciation of buachaill, it is the "i" that gets pronounced with the "a" making the "ch" broad [xɪl̪ʲ] and not the "a" that gets pronounced with the "i" making the "ll" slender [xal̪ʲ~xəl̪ʲ]? It seems like both the "a" and the "i" have an equal opportunity of being pronounced in the second syllable "buachaill," correct?
I don't know about pronunciation for your second question, but regarding the grammar for your first question: no.
It looks like they really condensed the information about the copula in the Tips and Notes for Basics 1. They used to have more details about it. For those who don't have any Tips and Notes:
Most sentences in Irish are VSO, or Verb Subject Object, although with the stative
bí it's really more VSC, Verb Subject Complement.
Itheann an fear bia | The man eats food (literally
Eats the man food)
Ólann sí bainne | She drinks milk (literally
Drinks she milk)
Tá sí ard | (literally
Is she tall)
But with the copula (which is a defective verb), the syntax is different. It's VCS, Verb Complement Subject. Just as in English you wouldn't say "A boy am I" (unless you're being poetic, but that's a rare exception and not the norm) but rather "I am a boy", in Irish it's always "Is buachaill é" and never "Is é buachaill".
It seems to me all verbs behave like that, at least in the habitual present tense which is all I've learnt so far. The verb form is the same for all persons, but in the first person, the pronoun merges with the verb to create a synthetic form in regular verbs, and it doesn't do that with "is". The difference seems to be that with "is", the object comes before the subject. And some of the pronouns drop their first letter. I mean "Is buachaill é" as opposed to itheann sé arán / Ithim arán. Sorry if I got any of this wrong...
A real verb (not is) gives the sentence the pattern: Verb Subject Object, but the copula (is and its forms) imposes a different pattern: Copula Complement Subject. The important thing to remember is that most Irish sentences have real verbs, but copula sentences are distinctly different.
The good news is that these 2 sentence patterns account for approximately every single Irish sentence ever.
Basically, it is used only when you want to say that the following two words are equal and refer to the same person or thing. Otherwise, use "tá" http://www.irishgaelictranslator.com/articles/grammar/ta-and-is-the-to-be-verbs/
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EDIT: The distinction is different than it is in Spanish.
It's like the Spanish
Spanish "estar", equivalent in Irish
Spanish "ser", equivalent in Irish
is … mé
is … tú
is … é
is … í
is … muid/sinn
is … sibh
is … iad
Irish is an Indo-European language.
Is "is" like "desu" in japanese?
Only in that it does not have different conjugations for I, you, he/she/it, we, y'all, they.
Irish grammar is Verb Subject Object the way English grammar is Subject Verb Object.
"Is buachaill é" word-for-word is "Is boy he".
If I wanted to say "She is a girl" that would be "Is cailín í", literally "Is girl she".
If I wanted to say "I am a woman", that would be "Is bean mé", literally "Am woman I".
There's nothing redundant about it. Irish just has different word order than English does.
English is subject-verb-object. Irish is verb-subject-object, or in this case, verb-complement-subject.
Word-for-word, this is literally "is boy he".
I'm not sure what you mean by "gender placeholder for the gender". Irish is verb-subject-object, or in this case, verb-complement-subject. https://www.duolingo.com/skill/ga/Basics-1/tips-and-notes
Word-for-word, this is literally "is boy he". Adjusting for the differences in Irish vs English grammar, this is exactly equivalent to "He is a boy". Would you call "he" a "placeholder"?