ku-

The ku- prefix is essential for talking about actions as concepts. Here is how it works:

• Meaning: It translates to both the English infinitive ('to read') and the gerund ('reading'). For example, kusoma means both 'to read' and 'reading'.
• Noun Class 15: When you add ku- to a verb, it becomes a verbal noun belonging to Infinitive Class 15. If you describe the action, the adjectives must take the ku- prefix (e.g., kusoma kuzuri = good reading). As a subject, it takes the subject concord ku- (e.g., Kusoma kunachosha = Reading is tiring), and for possessives, it uses kw- (e.g., kusoma kwangu = my reading).
• Phonology: It attaches directly to the verb root (ku- + cheza = kucheza). Before vowel-starting verbs, it generally stays as ku- (e.g., kuimba = to sing), but for a few specific verbs—-enda (to go), -isha (to finish), and sometimes -anza (to start)—it changes to kw- (e.g., kwenda).
• Short Verbs: Swahili requires a specific rhythm. For verbs with only one syllable in their root (like -la 'eat' or -nywa 'drink'), the ku- is kept in many tenses to maintain this rhythm (e.g., ninakula = I am eating). It is also kept in the imperative (command) form (e.g., Kula! = Eat!, Kunyweni! = Drink!). However, it must be dropped in the present negative (e.g., sili = I do not eat), past negative (e.g., sikula = I did not eat, where the ku is the tense marker, not the infinitive), perfect negative (e.g., sijala = I have not eaten), the subjunctive (e.g., nile = let me eat), and certain affirmative tenses like the -ki- conditional (e.g., nikila) and -ka- consecutive (e.g., nikala).
• Negative Infinitive: To form the negative (e.g., 'not to read' or 'not reading'), insert the infix -to- after the prefix, creating the Negative Infinitive with kuto- (e.g., kutosoma). Note that monosyllabic verbs retain their original ku- after kuto- (e.g., kutokula = not to eat).
• Object Markers: If you need to include an object marker, it is placed directly between the ku- prefix and the verb root (e.g., kumpenda = to love him/her). Crucially, if an object marker is added to a finite short verb, the dummy ku- drops because the object marker provides the necessary syllable for rhythm (e.g., ninakila = I am eating it, not ninakukila).

Note: Do not confuse this prefix with the Past Negative -ku- (e.g., sikusoma = I did not read) or the Second-Person Object -ku- (e.g., ninakupenda = I love you). Those are different markers that happen to look the same!