Breakdown of Судя по его поведению, он должен скоро уйти.
он
he / it (for a masculine noun) / it (for a neuter noun)
скоро
soon
должен
must, have to
уйти
to leave (on foot, pf) / to retreat into solitude
Questions & Answers about Судя по его поведению, он должен скоро уйти.
How does судя по work grammatically in this sentence?
The phrase судя по translates to 'judging by'. It is made from the verbal adverb of судить (to judge) plus the preposition по. It always requires the Dative case, which is why the neuter noun поведение (behavior) takes the Dative ending -ю to become поведению.
Since поведению is in the Dative case, why doesn't the possessive pronoun его change to match it?
The third-person possessive pronouns его (his), её (hers), and их (theirs) are invariable. They are originally Genitive forms of personal pronouns that became frozen. Unlike other possessives like мой or наш, they never decline to match the case of the noun they modify.
Why is должен used here instead of a regular verb for 'must' or 'has to'?
Russian does not have a direct verb equivalent for 'must'. Instead, it uses the short-form adjective должен (obliged), which changes by gender and number to match the subject. Here, it takes the masculine singular form to match он.
Could I use the imperfective verb уходить instead of the perfective уйти here?
Using уходить would change the meaning to a habitual action (he has to leave often) or a continuous process. Since we are talking about a single, specific departure in the near future (скоро), the perfective уйти is the correct choice.