Da / soH / bI / Su Difference?
I am seriously confused of the difference between these four Klingon words.
When I am practicing this course, it is hinted that these four words all translate to "you".
I understand that "bI" is used to address a single individual, "you are my friend". I also understand that "Su" is used to address multiple people. "You are all my friends".
bI-: verb prefix for you (singular) subject, no object
Da-: verb prefix for you (singular) subject, third-person object
SoH: you (singular) independent pronoun
Su-: verb prefix for you (plural) subject, no object
So using the example of German, SoH is like du (but without suggesting informality or familiarity), while the verb prefix bI- is comparable to the verb ending -st.
However, unlike European languages, Klingon verbs also inflect for the object of the verb. So if the verb has a third-person object (he/she/it), then it takes Da- instead.
Also, like in Italian, the independent pronouns can be omitted and usually are. The major difference in Klingon is that pronouns are also used to mean "to be," so SoH ends up doubling as "you are" and ends up appearing often in this context.
I've definitely seen lots of Klingonists say to just think of SoH as "(du) bist" rather than "du," i.e. a form of Klingon's irregular verb "to be." The biggest pitfall to this is that then you have to also remember that the subject of this irregular verb takes the -'e' ending without really understanding why... but different things for different people.
The closest equivalent to this on Duolingo is in Hebrew, as far as I'm aware (there may be something closer that I don't know about), where third-person pronouns often are used for "to be."
There are a bunch more of these prefixes, but maybe focus on getting these down for now.
I hope that helps. Someone will probably be along with a better explanation.
Your explanation is good.
"I've definitely seen lots of Klingonists say to just think of SoH as '(du) bist' rather than 'du,' i.e. a form of Klingon's irregular verb 'to be.'"
No, don't do that. SoH is not a form of "to be." It is not an irregular verb. It is only a pronoun meaning "you."
It just so happens that Klingon has an irregular sentence construction in which a noun (or noun phrase; from now on when I say noun, read "or noun phrase" into it) is linked to a pronoun to identify it as that pronoun. So tlhIngan SoH means that I'm taking the noun for "Klingon" and associated it with "you." When I talk about "you," I'm talking about a "Klingon." Now it just so happens that in English and other European languages, this linkage is performed by the verb "be" in all its various forms. But that does not mean that SoH equals "you are" or anything like that. It just means that the translation of noun-pronoun in Klingon comes out as pronoun-be-noun in English.
Klingon also extends this sentence construction in the third person to link two nouns. To do so, you have to choose one of them as the topic of the link. For example, tlhIngan ghaH HoD'e' The sentence is about the captain (the topic, marked with -'e'), and it links "him" ghaH with the "Klingon" tlhIngan. Again, it just so happens that European languages use a different sentence construction using "be," but that still doesn't mean Klingon pronouns are a form of "be." It just means that Klingon noun-pronoun sentences are usually translated as English or other European languages "be."
The fact that Klingon pronouns can take verb suffixes in such constructions is irrelevant. The entire construction is irregular; it is not a "basic sentence" as described in The Klingon Dictionary. You just follow the formula because that's what you do.
So... pronouns are pronouns. Their primary function is to replace nouns. In Klingon they have a secondary function of forming a copula, a linkage of nouns and pronouns. But SoH means "you," not "you are." SoH is not a complete sentence. Likewise for the other pronouns.