со-

Etymology
.

Prefix

 * 1) A prefix indicating unity or connection, akin to co-.
 * ученик
 * соученик
 * student
 * classmate
 * работа
 * соработка
 * work
 * cooperation, collaboration
 * 1) A prefix indicating composition, akin to syn-.
 * стави
 * состави
 * put
 * put together, assemble

Etymology 1
(see and 🇨🇬 for more information). The sound -ъ- was treated as strong when another yer followed in the next syllable, and was vocalized to -o-, whereas the yer in the next syllable was deleted, producing a consonant cluster.

Etymology 2
, from ; see above.

Prefix

 * 1) with, con-, co-