Lexicalized Causatives

In Swahili, the causative extension is highly productive, but many derived verbs have become lexicalized. This means their meaning has shifted away from a literal 'make someone do [verb]' to something more specific.

Transitivity:
Syntactically, the causative extension increases a verb's transitivity by adding an object. It turns intransitive verbs into transitive ones, and transitive verbs into ditransitive ones.

Morphological Forms:
The causative extension attaches to verb roots following strict rules. To understand these rules, it helps to know that historically there was a 'short causative' (a -y- sound that caused consonant mutations) and a modern 'long causative' (-isha / -esha). This explains why irregular mutations exist alongside regularized forms:

• Vowel Harmony: Roots with a, i, u take -ish-; roots with e, o take -esh-.
• Consonant Mutation: While many regular verbs ending in -ka or -ta simply take the standard suffixes (e.g., pika > pikisha; kata > katisha), historically, some mutated directly to -sha or -sa. This is especially true for verbs where -ka is a stative extension (e.g., choka > chosha) or as a historical relic (e.g., amka > amsha), as well as roots ending in -t (e.g., pata > pasha; takata > takasa). Note that pasha is highly lexicalized (e.g., pasha moto 'to warm up'). Some verbs with a root ending in -n have a short-causative alternation yielding -ny- before final -a (e.g., pona > ponya).
• Regularized Doublets: When a mutated causative becomes highly lexicalized, Swahili often creates a new, regularized causative to handle the literal meaning. For example, because pata (get) mutates to the lexicalized pasha, the regularized patisha is used for 'cause to get/experience'. Similarly, pita (pass) mutates to pisha (make room/let pass), while pitisha is used for 'cause to pass' (like passing a law or an object).
• Vowel-Final Roots: For native Bantu verbs ending in double vowels, the standard causative mutation is usually -z- (e.g., jaa > jaza; potea > poteza). The -lisha or -lesha forms are typically causatives built on top of the applicative extension, or they are used for monosyllabic verbs (e.g., kula > kulisha) and some loanwords (e.g., sahau > sahaulisha).
• Arabic Loanwords: Arabic loan verbs that do not end in -a often replace their final non-a vowel when forming -isha or -esha; learn the attested derived stem (e.g., rudi > rudisha; jibu > jibisha).
• Double Causatives: Some verbs take a double causative suffix for historical reasons, which explains common words like ona > onyesha (from ona > onya > onyesha). Note that penda > pendeza uses a single causative suffix -ez-.
• Deriving from Adjectives and Nouns: The causative extension is highly productive for creating verbs from non-verbs (e.g., safi > safisha 'to clean'; fupi > fupisha 'to shorten').

Why This Matters:
Lexicalized causatives are extremely high-frequency, everyday vocabulary words. You cannot simply guess their meaning on the fly. For example, soma means 'read' or 'study', but somesha usually means 'educate' or 'pay for someone's schooling', not literally 'force someone to read'.

Facilitation vs. Coercion:
Swahili causatives naturally express facilitation or direct causation (enabling or causing an event to happen). However, to express literal coercion or 'forcing' someone to do something, native speakers frequently use a periphrastic construction, such as kufanya mtu asome (to make a person read) or the verb kulazimisha (to force).