Breakdown of Этот рельс может быть сломан, он должен быть осмотрен.
он
he / it (for a masculine noun) / it (for a neuter noun)
этот
this [f: эта, n: это]
быть
to be
мочь
to be able to
Questions & Answers about Этот рельс может быть сломан, он должен быть осмотрен.
How does the passive infinitive construction work in this sentence?
We use a modal word (может or должен) followed by the infinitive быть (to be) and the short form of a past passive participle (сломан, осмотрен). In Russian, the passive voice is almost always formed using these short participles rather than their full-length adjective forms.
What determines the ending of the short participles сломан and осмотрен here?
They must agree in gender and number with the subject of the sentence, which is рельс (a masculine noun). Even though they come directly after the infinitive быть, they still take the masculine singular form to match рельс (and its pronoun он).
Notice how может and должен behave differently. Why does one conjugate like a verb while the other changes by gender?
Мочь (can/might) is a standard verb, so может is its 3rd-person singular conjugation. Должен (must), however, acts as a short adjective in Russian. It does not conjugate by person; instead, it changes by gender and number to match the subject. Here, it takes the masculine singular form to match он.
Why are these called past passive participles if the rail needs to be inspected right now or in the future?