For want of a nail the sentence was lost...
Some of the sentences are based on construction or farming, given that it's loosely following the Peace Corps syllabus. Huna misumari, huwezi inajenga, you have no nails, you cannot build it (probably incorrect Swahili, but you get the idea).
As an aside, nail is an M/Mi noun. This is because it is long and thin, just like a tree (also in this class) is long and thin.
Not quite. It's concepts. Some people call M/Mi the tree class. So you start with tree, mti/miti. A tree is a plant, mmea/mimea (fruit plants are mostly M/Mi, so if you see michungwa instead of machungwa, it isn't incorrect, its orange trees, not oranges). Some things made from plants also fall in this category.
Construction nails are in this class, as are spears (mkuki/mikuki), chains (mnyororo/minyororo) and umbrellas (mwavuli/miavuli), all long and thin, like a tree or a river (mto/mito).
Some of nature is also in this class like fire (moto/mioto), and active things, including various body parts like heart (moyo/mioyo) and hand (mkono/mikono).
Then there are words which fit with these concepts that are part of another noun class, so it's not set in stone (stone, by the way, might be considered nature but isn't M/Mi. It is Ji/Ma, jiwe/mawe, because it also fits with a concept in that class). Hopefully someone with more understanding about this aspect of the language will come along with a better explanation.