"I go to karaoke with my friends on Friday."
I am still having trouble with this basic concept of just WHAT is the subject. It seems like the last think I would think of is that "Friday" is what it is all about! Yes, I can read this as "Speaking of Friday, ...", but it seems just as reasonable to make "I" the subject, or "I with my friends."
"I" is the subject of this sentence, you can tell because it is doing the action. The first-person pronoun「私」(or one of its many alternatives) has been dropped from the sentence since it's clear what the subject is from the context, and Japanese sentences do not require a subject as English sentences do.
「金曜日」is the topic of the sentence, not the subject: these are distinct grammatical concepts. The subject is marked with the particle 「が」, the topic is marked with the particle 「は」. All the topic is really doing is specifying what the rest of the sentence is about.
As to why「金曜日」is the topic here, we actually don't know as the sentence itself lacks any context. Maybe the speaker had been talking about what he does on Thursdays and needed to change the topic so that it could be understood that he was now talking about Fridays. But without context there's actually no reason why「金曜日」has to be the topic, the following sentences are all grammatical (though some are potentially very weird).
「金曜日、友達とカラオケに行きます。」 「金曜日、私は友達とカラオケに行きます。」 「金曜日、カラオケには友達と行きます。」 「金曜日、友達とはカラオケに行きます。」
Just like the original sentence, these can all be translated as "I go to karaoke with my friends on Fridays" but the nuance is changing. By marking something as the topic, you are emphasizing what follows. So in the original sentence, you've established that you're talking about Fridays, now the important information is that you're going to karaoke, and doing so with your friends.
[BlackYeti42 - love the moniker ]
First off, thanks. In this particular case, the subject "I" is probably fairly obvious. Then, if I were just trying to decide (without additional context), I would say that going to Karaoke is the topic. Obviously, here, Duo disagrees.
In lots of other sentences, however, Duo relegates the day or yesterday or "last week" to a mere passing thought. Maybe with a に and more often with nothing at all. So the translation here makes Friday so important that it is the topic. I guess my frustration is not with the fact that here Friday is the topic (as you say, I can see cases where that would be true), but rather - How the heck are WE supposed to know??? English does not provide that information, and this is an English -> Japanese translation question. Sigh...
Again, I sure appreciate the assistance ...