Grammatically, ya is used when habari is singular and za is used when it is plural. The noun habari itself doesn't change.
As for what the difference in meaning is, I'm not sure. Perhaps using za (and therefore the plural of habari, "reports, messages, pieces of information, pieces of news") invites a little more detail than using ya, which, in my mind, would make sense for asking for one piece of information. This is just some conjecture on my part though and I wouldn't trust it till a native speaker or competent user of Swahili has given input.
It's a bit arbitrary which from you choose, but zako is plural when it comes to the pieces of news, and singular when it comes to number of persons asked. You would use zenu when greeting several persons, and if you greet someone who you haven't seen in a long time, or someone who you haven't met before, it is common to use the 'z-' forms, implying that she or he has many 'pieces of news', because many days have passed.
The important thing to remember is that this is not a question about what is actually happening in the life of a person, but a greeting. The answer would still be "Nzuri sana!", and then you could start telling about what has happened since last time, and start asking more specific questions. Those questions could also be part of an extended greeting. And the better you know a person, the longer the greting phase would be. For example, if the person you are greeting has a wife, that you also know, or he has talked about the last times you met, it would be natural to ask "Habari ya mkewe?", and the answer is still "Nzuri sana!".
Shouldn't that be "when it comes to the pieces of news, za is plural (several pieces of news) and ya is singular (one piece of news)"?
As you explain, and several other comments here confirm, the amount of news the speaker indicates (singular or plural) depends on the number of people asked (or asked about) and how much news they can be expected to have (that you don't already know about). So:
- ya (singular) for one person you see often
- za (plural) if you haven't seen this person for a long time
- za (plural) if you are meeting someone for the first time
- za (plural) if you are asking several people (even if you see them often).
I am not questioning that it is a greeting and that the answer is always "nzuri" or similar (even if you have some bad news to tell). The real question is how can this be translated to an English greeting that any English speaker would say and understand as an equivalent greeting.
"How is the day?" is completely meaningless, which is why so many people have commented here that they can't work out what these three simple words mean when put together.
Maybe "Habari ya leo" would make sense if translated as "How are things today?"
"Habari ya mkewe?" would then translate as "How are things with your wife?)
@Catriona, yes 'za' for several pieces of news and 'ya' for one piece, and it is used as you say, 'za' with several persons (although it is more common to greet persons one at a time), or with someone who you haven't seen in a long time, and 'ya' for one person, usually.
But my response was to @AGreatUserName and to @hussain14656 who mentioned 'zako' and 'yako'. They also work like 'za' and 'ya', except you would never say "Habari zako!" to several persons at once. The word '~ako' is a possessive used when talking to one person, and it means 'your'. If you are talking to several persons, and want to say 'your', you would use the word '~enu'.
Interesting! Our Tanzanian teacher just taught us to say habari yako (not zako) to one person but habari zenu (not yenu) to several people. So maybe he thinks of it as one piece of news per person.
And @hussain14656 - maybe only one piece of news because it isn't that long since your parents last greeted you? Just a thought ...
But note that "Habari yako" is a separate phrase (literally "Your news?").
This question is "Habari ya leo?" and it looks like it's more normal to say "Habari ZA leo?", matching other native-speaker comments here and the other questions in this lesson:
"Habari ZA asubuhi?"
"Habari ZA mchana?"
"Habari ZA jioni?"
"Habari ZA usiku?"
Yes, but almost no one says "habari YA leo", it is more common to hear people say "habari ZA leo. They use habari ZA leo for plural or singular, you are right, it is correct to say habari YA leo for singular and ZA for plural, but when speeking, it does not really mater what you say!
Habari is neither plural nor singular on its own. Or more precisely, it is both. But, in my experience, it is, as you said, more common with 'za' in this case, with a stronger preference to the 'z-' forms (za ..., zako, zenu) when it's a long time since last time you spoke.
And my experience is from Kenya and with Kenyans. When I was a student, I shared an apartment with some Kenyans for several years, and later I've also got to visit them in Nairobi and Kitui. I don't know much about Tanzania, and what the preferences are there, but it is probably quite similar.
Technically, this is correct, but the way the sentence is used, the translation above isn't that bad. Both are ways of greeting, and the common way to respond is to say the news/day is good without any more qualifications. If you really want to ask what's new, then you would have to ask in a different way.
They are both very general questions that are just greetings, very generally asking how you are. One technically is "Have you been victorious?", the other "(What is the/your) news of today?" Both should be answered, "Njema", just like the generic, "How are you?" should always be answered "Fine."
But both are generic greetings that are in a very general way asking about the condition of the one being greeted, and both are always answered with the generic "Njema." So even though the questions technically are different, the use is basically the same: a generic greeting. To me, the useful (not techinical) English equivalent of both is, "How are you?"
Literally, "habari" means news and "leo" means today. But it's about that person's news, not about what's in the newspaper. It is a greeting, and I think a more natural equivalent in English is "How are things today?"
You could say it to anyone, not just informally to friends. (See today's comments further up this thread.)
Since in the use of greetings, no one is interested in the general news of the day (ei: newspaper news, etc., or wondering about the weather that can be observed right then, that day), it is expected that this greeting is showing interest in the PERSON being greeted, in THEIR news of that day. "How is the day?" is rather meaningless in English at all, and especially as a greeting! ??? I think that "How are you today?" should be accepted as an English translation of this Swahili greeting.
I agree, MGB266009. Native English speakers just don't say "How is the day?" so it shouldn't be the recommended answer.
Better to choose something idiomatically equivalent, but that means that the course developers will have to add a lot of alternative answers and students will have to dare to submit answers including "you" or "your" even though this isn't in the Swahili original. They just have to be sure that their suggested greeting (like yours) would fit with the response "Fine!"