。
Ai sat down beside me.
Literal
Ai-[diminutive-ちゃん] [topic-は] me [of-の] side [at-に] sat.
A simple declarative narrative sentence. ~ちゃん is the affectionate/diminutive suffix attached to personal names — typically for children, close friends, or affectionate reference to women. そば 'side, vicinity' is a native Japanese word for 'beside,' distinct from the Sino-Japanese 近く. 座る 'to sit down' is the change-of-state verb for the act of sitting; に marks the location of sitting.