When the subject of the sentence is plural, the verb (more accurately the predicate) can be singular OR plural.
As a rule of thumb, "non-human subjects" almost always take a "singular" verb. Human subjects on the other hand, can take a singular or plural verb. "Usually" if the action is completed by the subjects as one unit, the verb is singular.
But these rules are not very strict any more, therefore Duolingo should accept both "okur" and "okurlar".
Both A1fie and LupoMikti's replies were very good. In fact, they are some of the most informative replies I've seen on this topic. I don't have much to add, other than a couple of memory aids and a comment about how this relates to English.
As a memory aid, you could think of this grammatical structure in this way: When you do things altogether as one, you are often more efficient. Efficiency often means streamlined and the streamlined version of something eliminates anything unnecessary, to include the –lar/-ler suffix.
You could also think of it this way: In English, when we refer to a bunch of people as a group, we apply the singular forms to it. We say, “The group is leaving,” and not “The group are leaving.”
Having said that, I’m kind of hung up on the fact that if a bunch of people were acting as one, as in a group for example, wouldn’t you express that with an appropriate choice of words rather than simply eliminate a suffix? [For example, say, "Group of them/people ..." rather than just "They (onlar) ..."] Let me apply this to an actual sentence to further illustrate:
Grup gidiyor. (Literally, "The group is going.")
Onlar gidiyor. [Literally, "They is going," but translated into English as "They are going." The implied meaning, in English, seems to be "They (as in the group of them) is going and therefore the third person singular verb form is used. Translated literally, it would be "They is going," which would not be grammatically correct and used only in rare, colloquial instances.]
If Google search results are a good indicator, it appears that "onlar gidiyor" is clearly the preferred way of conveying this.
On a side note, in English, we could do something similar with a third person plural subject -- use third person singular (3PS) instead of third person plural (3PP) -- but it would be considered bad English. For example, we could say, “The girls is leaving,” but it would be an affected, cutesy sort of way to say “The girls are leaving.” The first version is not grammatically correct and would be rarely heard whereas the second is grammatically correct and the way you would hear this conveyed most often. I get the impression, from reading several posts, that the distinction between 3PS and 3PP isn't quite as rigid in Turkish as it is in English.
Hope that helped in some way.
I am Turkish and i can say that is wrong to say okurlar. It is gramatically wrong. It should be "okur" . With plural object , verb supposed to be always singular. However it is not a big deal to say "okurlar " Even the most Turkish people do the same mistake. But in duolingo they teached you in this way. As i said it is not a big deal to say "okur" or "okurlar" . They should be both accepted.
Edit: what i said works with they (onlar ) . not with siz ( you plural) . For example if you say:
"You(pl) read the book" ==>Siz kitabı okursunuz, not "siz kitabı okursun."
So, let's say that there's a classroom full of students all assigned to read the same book. Each gets their own individual copy of that book, but the reading of the book will only be done during class, not on their own.
Now, if I wanted to say "They read the book", would I use the singular okur or the plural okurlar? Thanks to the ambiguities of English, I could reason that because all of the students make up the unit that is the 'class', they are doing the activity of reading the book together, and could use okur. However, I could also reason that because each student individually reads the book in the class, I should use okurlar. In both instances the students all read the same book as a class, but they have to physically read the book individually.
In this situation, what would Onlar kitabı okurlar signify as opposed to Onlar kitabı okur?
In the last question 'Onlar gazete okur' was translated to 'they read newspapers' and the reason given for translating 'gazete' into 'newspapers' instead of newspaper was that it was not written onlar 'bir' gazete okur. But here, its also not written bir with the kitabı. But it translates into the book instead of the plural form. Can someone please clarify?