toisiaan

The reciprocal pronoun is built from the word toinen (other/second). In its reciprocal sense, it follows these rules:

• Morphology & Agreement: It usually consists of the plural stem toisi- + a case ending + a possessive suffix. The suffix must match the person of the subject. Because reciprocity inherently requires at least two people, the reciprocal pronoun takes suffixes matching the plural participants: -mme for we, -nne for you plural, and the third-person suffix for they. The third-person suffix itself does not distinguish singular and plural. Forms like toiseni or toisesi do not exist in this sense. Note that when adding a possessive suffix to a case ending that ends in -n, the final consonant is deleted (e.g., toisiin + nsa = toisiinsa). In the 3rd person, the suffix -nsa/-nsä often appears as a lengthened vowel + n (e.g., toisi-a-an).
• Subjectless & Collective Sentences: In sentences without an explicit subject (such as passive or generic sentences) and with collective singular subjects, the 3rd person suffix (-nsa/-nsä or -an/-en) acts as the default.
• Partitive Object Forms: Because many common verbs of interaction take the partitive case, the partitive plural forms (toisiaan, toisiamme) are the most common default forms.
• Total Object Forms: For a total object (e.g., with tuntea meaning know), start from the nominative-shaped plural toiset, where the final -t is deleted when a possessive suffix is added: toiset + mme = toisemme, toisenne, toisensa (e.g., Me tunnemme toisemme).
• Genitive Forms & Postpositions: The plural genitive form toistensa (each other's) is most common, derived from the -ten ending, though toisiensa is also a valid alternative. This genitive form is frequently used with postpositions (e.g., toistensa kanssa, with each other).
• Case Selection: The case is determined by the verb. If the verb takes the illative, use toisiinsa.

Advanced Variations:
1. toinen toistaan / toinen toisiaan: In this doubled construction, the first toinen remains in the 'frozen' nominative singular. The second part can be either singular (toistaan) or plural (toisiaan), both of which are correct and accepted in modern Finnish. Only the second word takes the possessive suffix matching the subject; do not attach it to the first toinen. The suffix on the second word agrees (e.g., Me luotamme toinen toiseemme, He luottavat toinen toisiinsa). This doubled structure is completely synonymous with the single plural structure (e.g., toisiimme, toisiinsa) for basic verbs, but is slightly more emphatic or formal.
2. Comparisons: The doubled form is often used for emphasis in comparisons, e.g., toinen toistaan parempia (each better than the last).

Other Ways to Express Mutuality
Keskenään means among themselves or with one another: He puhuivat keskenään. Use keskenämme with we and keskenänne with you plural. It is an adverbial alternative in suitable contexts, not a replacement for every case-governed reciprocal object. Some grammars call total-object forms accusative by function; the lesson uses nominative-shaped to identify the form without requiring that terminological convention.