Breakdown of Он медленно сел в кресло.
он
he / it (for a masculine noun) / it (for a neuter noun)
медленно
slowly
в
in / to (direction)
кресло
armchair
Questions & Answers about Он медленно сел в кресло.
Why do we use the accusative в кресло instead of the prepositional в кресле?
Because сесть (to sit down) is an action of assuming a position, which Russian treats as motion towards a destination. It answers the question "where to?" (куда?). You use в or на + Accusative. Once he is already seated and resting there, you use the static verb сидеть with the prepositional case: Он сидит в кресле (He is sitting in the armchair).
The lesson is about reflexive verbs (ending in -ся). Why doesn't сел have a reflexive ending?
That is an important catch! The imperfective verb for "to sit down" is reflexive: садиться. However, its perfective counterpart сесть is an exception—it is not reflexive. So you will never see a -ся on its forms like сел.
Why do we say в кресло but на стул (on a chair)?
It depends on the shape of the furniture. An armchair (кресло) has depth and armrests, so Russian treats it as a space you sink into (в). A standard chair (стул) or a sofa (диван) is treated as a flat surface you sit on top of (на).
How do we form the past tense of сесть?