The lexical derivation of verbs in Swahili follows certain patterns to create terms that may semantically seem more like inflected forms, but have their own independent inflections. The morphemes used to create these patterns are known as verbal extensions. Derivative verbs may frequently have special, unpredictable lexical meaning and are sometimes formed irregularly. Not all possible derived forms will exist, and sometimes multiple forms will exist for a single derivation, even with different meanings. Extensions may also be combined where logically possible. Some verbs even lack a base form and only exist as derivative verbs, especially for the nonproductive extensions. This is simply a guide to the regularly formed derivative verbs.
Please refer to Appendix:Swahili verbs for background on verbal inflection, and the conjugational paradigms that will be referred to here.
Several verbal extensions contain a vowel subject to vowel harmony, which will be written as <E> below. This vowel can be analysed as being an underlying <e>, which harmonises to become <i> when the vowel most closely preceding it is <a>, <i>, or <u>. When the preceding vowel is <e> or <o>, it appears as <e>, and when there is no preceding vowel, it is unpredictable.
The passive voice, which can only be used for transitive verbs. Normal verbs ending in a consonant followed by -a change the -a to -wa, and normal verbs ending in two vowels usually take the extension -lEw-. Only four monosyllabic verbs can be passivised; their forms are -liwa (from -la), -chewa (from -cha), -nywewa (from -nywa) and -pewa (from -pa; dialectally -pawa). Arabic verbs ending in -i or -u change the final vowel to -iwa, those ending in -e add -wa to the end, and those ending in -au add -liwa to the end.
Sometimes also called the prepositional, the applicative voice has a variety of uses but always serves to promote an oblique argument to being the core argument. It may sometimes be benefactive, malefactive, instrumental, or locative in sense. Normal verbs ending in a consonant followed by -a change the -a to -Ea, and normal verbs ending in two vowels usually take the extension -lE-. Monosyllabic verbs are entirely unpredictable. Arabic verbs ending in -e or -i add -a to the end, those ending in -u change it to -ia, and those ending in -au add -lia to the end.
The stative expresses that something has entered or may potentially enter into a state without an actor. Normal verbs ending in a consonant followed by -a change the -a to -Eka, and normal verbs ending in two vowels usually take the extension -lEk-. Only three monosyllabic verbs can be made into a stative; their forms are -jika (from -ja), -lika (from -la), and -nyweka (from -nywa). Arabic verbs ending in -i or -u change the final vowel to -ika, those ending in -e add -ka to the end, and those ending in -au add -lika to the end.
Causative forms are much more intricate, and often a regular and irregular derivation will coexist for a single verb. Normal verbs ending in most consonants followed by -a usually change the -a to -Esha, and normal verbs ending in two vowels usually change the -a to -za. Normal verbs ending in certain consonants can sometimes exhibit unusual forms: those where the last consonant is <k> may change it to <sh>, those where the last consonant is <t> may change it to <s> or <sh>, those where the last consonant is <n> may change it to <ny>, and those where the last consonant is <w> may change it to <vy>. Monosyllabic verbs are entirely unpredictable. Arabic verbs, which can often be formed from nonverbal Arabic roots as well as Arabic verbs, usually replace the final vowel with -Esha, with those ending in -au adding -lisha to the end.
Reciprocal forms are very simple, just adding -na to the end. For Arabic verbs, the reciprocal cannot be applied to the base verb, and instead must be applied to the applicative form. (Other verbs may distinguish a reciprocal from an applicative reciprocal.)
Static verbs describe a stationary condition, and constitute a nonproductive extension. It is formed by adding -ma to the end, and in some cases may only exist as a reciprocal static.
Conversive verbs make the meaning opposite from that of the base verb, and constitute a nonproductive extension. It is formed by replacing the final vowel with -ua for all verbs except those where the preceding vowel is <o>, in which case -oa is used instead.
Augmentative verbs usually intensify a verb or modify its meaning, and constitute a nonproductive extension. It is formed extremely irregularly, often following the same rules as the conversive extension, in which case no conversive form exists.
Reduplicative verbs usually indicate repetition over a short time scale, and constitute a generally nonproductive extension. It is usually formed by reduplicating the entire verb stem, but a handful of verbs simply reduplicate the first syllable of the stem.
Verbs may also be derived to form nouns, with agent nouns replacing the final vowel with the suffix -i and going into class 1, and other derivational nouns replacing the final vowel with -o and going into a variety of classes, especially 3, 5, 6, and 7. See Appendix:Swahili noun classes for more on these.
Nominal derivations can be made from any of the derived verbal forms listed above. Many nominal derivations may seem strange or irregular from a synchronic perspective, because they are in fact inherited from Proto-Bantu and may exhibit spirantisation or other unexpected sound changes.