A Swahili ku- infinitive is both verbal and nominal. It can retain a verb's object and modifiers, yet the whole expression belongs to noun class 15: Kusoma vitabu kunapanua fikra “Reading books broadens the mind.” English alternates among to read, reading, and abstract nouns; Swahili often keeps one class-15 form.
The important diagnostic is external agreement. When the infinitive phrase controls another word, class-15 ku- appears again on that dependent element.
This is a mixed category, and that is genuinely harder than the English label “infinitive” suggests. Inside the phrase, the form behaves verbally: it can select an object and take an adverb. Outside the phrase, it behaves nominally: it occupies noun positions and controls class-15 concord. The class-15 agreement system makes that dual behavior visible.
One form, several sentence functions
| Build | Finished expression | Function |
|---|---|---|
| ku-som-a | kusoma | reading / to read |
| ku-pik-a chakula | kupika chakula | cooking food |
| ku-i-som-a ripoti | kuisoma ripoti | reading the report |
| ku-to-chelew-a | kutochelewa | not being late |
| ku-som-a ku-na-saidi-a | kusoma kunasaidia | reading helps |
| ku-safir-i ku-li-chosh-a | kusafiri kulichosha | traveling was tiring |
| ku-fany-a kw-ake | kufanya kwake | his/her doing |
| ku-imb-a ku-zuri | kuimba kuzuri | good singing |
Internal ku- builds the infinitive. External ku- in kunasaidia or kulichosha is class-15 subject concord. Kwake is class-15 associative kwa plus possessive -ke. The repeated sound reflects agreement, not redundant wording.
Infinitive phrases as subjects
Kusoma kwa sauti kunawasaidia watoto kutambua maneno mapya.
Reading aloud helps children recognize new words.
The subject is the entire phrase kusoma kwa sauti. It triggers ku-na-wa-saidi-a, including class-15 subject ku- and human plural object wa-.
Kutembea kila asubuhi kumeboresha afya yake.
Walking every morning has improved her health.
Perfect ku-me-boresh-a agrees with the activity. English treats walking as singular but shows that only in has; Swahili identifies the noun class explicitly.
Kutolala vya kutosha kunaweza kupunguza umakini kazini.
Not sleeping enough can reduce concentration at work.
Negative infinitive ku-to-lal-a remains a class-15 noun phrase and controls ku-na-weza. Its negation is internal to the activity, not the negation of the main claim.
The nominal boundary leaves verbal grammar inside
Class 15 makes the whole infinitive phrase nominal; it does not strip verbal morphology from the infinitive itself. Objects, object markers, extensions, reflexive ji-, negation, and passive morphology can all remain inside that boundary:
| Internal feature | Analytic build | Finished form or phrase |
|---|---|---|
| direct object | ku-som-a ripoti | kusoma ripoti “reading a report” |
| object marker | ku-i-som-a | kuisoma “reading it” |
| applicative extension | ku-som-e-a watoto hadithi | kusomea watoto hadithi “reading the children a story” |
| reflexive marker | ku-ji-funz-a Kiswahili | kujifunza Kiswahili “learning Swahili” |
| internal negation | ku-to-tum-a barua | kutotuma barua “not sending a letter” |
| passive extension | ku-fung-w-a | kufungwa “being closed/locked” |
| causative + passive | ku-chelew-esh-w-a | kucheleweshwa “being delayed” |
To locate the boundary, bracket everything selected by or modifying the infinitive: [kusoma ripoti kwa makini]. If that bracketed phrase is the subject, tense and aspect appear on the following finite predicate, which begins with class-15 subject ku-; the object ripoti cannot control that outer predicate. If the bracketed phrase is merely a complement after alianza or anapenda, it does not add agreement to the preceding finite verb. This two-step test—identify the internal verbal phrase, then identify its external sentence function—prevents agreement with a nearby participant.
As an object or complement
An infinitive can fill the object-like complement position after verbs of liking, beginning, avoiding, remembering, and many other meanings.
Ninapenda kupika kwa marafiki zangu mwishoni mwa wiki.
I like cooking for my friends on weekends.
Tulianza kujadili bajeti baada ya chakula cha mchana.
We began discussing the budget after lunch.
Meneja alikataa kusaini hati bila kuisoma.
The manager refused to sign the document without reading it.
The understood actor of each infinitive is controlled by the finite subject: nina- “I,” tu- “we,” and a- “the manager.” That is a syntactic relationship, not agreement written inside kupika, kujadili, or kusaini.
As an ordinary direct object, an infinitive phrase may also be discussed or evaluated:
Watoto wanapenda kuogelea, lakini wazazi wao wanaogopa kuwaruhusu peke yao.
The children like swimming, but their parents are afraid to let them go alone.
English chooses swimming after like and to let after afraid. Swahili does not copy that lexical alternation; both actions use ku- forms.
Predicate and identificational uses
After copular ni, an infinitive can identify a goal, task, problem, or recommended action.
Lengo letu ni kupunguza matumizi ya maji kwa asilimia ishirini.
Our goal is to reduce water use by twenty percent.
Kazi yangu ya kwanza ni kuwapigia wateja wote simu.
My first task is to call all the customers.
The infinitive does not agree with lengo or kazi internally. Ku- marks the action expression; any object marker follows it, as in ku-wa-pigi-a.
Evaluative predicates can instead take the infinitive as subject:
Kufika mapema ni muhimu katika mahojiano ya kazi.
Arriving early is important in a job interview.
This common pattern uses invariant predicate ni muhimu. When an agreeing adjective is chosen, class 15 appears: Kuogelea kuzuri kunahitaji mazoezi “Good swimming requires practice.” Such agreeing adjectival phrases are more (formal) and (academic) than simple everyday evaluations with ni vizuri or ni muhimu.
Possessors and modifiers
Because the infinitive is nominal, it can take class-15 possessive linking.
Kuchelewa kwake kuliwafanya abiria wengine wakose basi.
His lateness caused the other passengers to miss the bus.
Kwake means “his/her” with the class-15 connector kw-. The main past verb is ku-li-wa-fany-a: class-15 subject, past, human plural object, stem.
Kuimba kwao kulisikika hadi mwisho wa barabara.
Their singing could be heard as far as the end of the road.
The possessor belongs to the whole event noun. It need not be the grammatical subject of a finite verb inside the infinitive.
Productive event phrases versus lexical nouns
An infinitive phrase is productively available whenever a verb and its dependents need to be treated as an event: kuandika taarifa “writing a report” still retains the object taarifa. A related lexical noun packages a more specialized meaning and has its own noun class: mwandishi/waandishi names an author or writer, uandishi a practice or style of writing, mwandiko/miandiko handwriting, script, or a manner of writing, and maandishi written material. The passive infinitive kuandikwa instead names the event of being written.
These forms are not interchangeable derivations. Choose the infinitive when the expression must remain compositionally verbal, especially when it keeps an object, extension, negation, or adverbial. Choose the attested lexical noun when the person, practice, style, product, or material has become the topic. Its plural and agreement then come from that noun's class, not from class 15.
Common Mistakes
Using human agreement for an activity subject
❌ Kusafiri usiku anachosha.
Incorrect — the subject is a class-15 activity, not a class-1 person.
✅ Kusafiri usiku kunachosha.
Traveling at night is tiring.
Translating every English -ing form with a finite present
❌ Ninaogelea ni mazoezi mazuri.
Incorrect for ‘Swimming is good exercise’ — ninaogelea means ‘I am swimming.’
✅ Kuogelea ni mazoezi mazuri.
Swimming is good exercise.
Giving the possessor the wrong class connector
❌ Kucheza yake kumewafurahisha wageni.
Incorrect — a class-15 infinitive takes possessive kwake.
✅ Kucheza kwake kumewafurahisha wageni.
His dancing has delighted the guests.
Negating the choice instead of the chosen activity
❌ Meneja hakuchagua kushiriki katika mjadala.
Wrong for ‘chose not to participate’ — this says the manager did not make that choice.
✅ Meneja alichagua kutoshiriki katika mjadala.
The manager chose not to participate in the discussion.
Key Takeaways
- A ku- infinitive can be a subject, complement, predicate expression, or modified action noun.
- As a subject it triggers class-15 ku- concord: kusoma kunasaidia.
- Its verbal interior may still contain objects and adverbials.
- English gerunds and infinitives frequently correspond to the same Swahili class-15 form.
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Start learning Swahili →Related Topics
- Forming the ku- InfinitiveA2 — Build Swahili infinitives with class-15 ku-, preserving stem-final vowels and recognizing the restricted kw- forms of vowel-initial verbs.
- Infinitives after Modals and Control VerbsB1 — Link weza, taka, penda, jaribu, and sahau to a same-subject infinitive, while distinguishing impersonal and person-marked lazima frames.
- Class 15: ku- Infinitive NounsA2 — Use Swahili ku- infinitives as verbal nouns, recognize their ku- agreement, and distinguish nominal infinitives from finite verbs and deceptive ku-initial nouns.
- Infinitive ku- as an Agreement ControllerB1 — Treat Swahili infinitives as class 15 verbal nouns when they control predicates, possessives, demonstratives, and modifiers.