First time I hear about kwenda as a preposition. Kwenye, kwa - to, towards...... This is why this sentense was strange for me. Kutembea - to walk + kwenda - to go ( go away ) for me doesn't work. By my opinion the sentense - Hatukutembea shuleni - is proper for given answer.
According to the Locatives unit tips:
Preposition Suffix -ni
In Swahili, some common nouns can be suffixed by –ni to form locations which mean: in, on, at, and to.
- Chumba will become chumbani – in the room
- Shule will become shuleni - at/to the school
So that suggests that "Hatukutembea shuleni" can mean "We didn't walk to (the) school".
(In fact, it just accepted "Hatukutembea shuleni" as the answer to "We didn't walk to school", also in this lesson.)
Is the sentence in this question trying to convey that we didn't walk to go to school (e.g. because we did go to school but we went by car)?
If you are using the Duolingo smartphone app you won't see any Tips&Notes. You have to go to https://www.duolingo.com/ on your browser app or PC. There, you click once on the lesson/unit in question then click on the light bulb icon instead of the Start button. But here is the direct link to Tips&Notes for locatives:
As I've read somewhere, Swahili and English have an interesting parallel kind of difference.
In English, we can say "Go to school/prison/hospital" etc., without an article and this indicates that we are going to the place for the main expected reason, ie. we are going to learn / be incarcerated / be treated there as a student / inmate / patient respectively.
If you work at one of these places or are going there for any other reason, you'd say "I'm going to the school" etc. (or "a"). <
In Swahili, I don't know how widespread the parallel is but I've seen it at least for shule(ni) and hospitali(ni). Of course, it's not an article missing but the choice between the locative variety of the noun and the original form of the noun.
Ninakwenda shule = I am going to school. (I am a student and I will learn there.)
Ninakwenda shuleni = I am going to the school. (No indication of what I will do there, maybe I work there or I am delivering something, visiting etc.)
I don't remember where I read this though, but if I find it again, I'll reference it here.