Breakdown of Я чуть не потерял свой проездной в метро.
я
I
в
in / to (direction)
метро
subway
свой
one's own
Questions & Answers about Я чуть не потерял свой проездной в метро.
Why is there a не in the Russian sentence if the translation says "almost lost" without any negation?
This is how Russian expresses an imminent averted action. The phrase чуть не (literally 'a little bit not') pairs with a perfective past tense verb to show that an action was on the brink of happening, but ultimately did not. The не shows that the actual loss of the pass was averted.
Does the verb after чуть не always have to be perfective, like потерял?
Yes, almost always. The structure чуть не (or едва не) specifically requires a perfective past verb because it describes a single, specific event that reached a critical point and stopped just short of completion. An imperfective verb wouldn't make sense for a sudden, averted close call.
The word проездной looks like an adjective. Why is it used as a noun here, and why didn't its ending change?
It is an adjective that has turned into a noun through everyday use, short for проездной билет (transit ticket). Since it functions as a masculine inanimate noun, its accusative form (acting as the direct object of потерял) is identical to its nominative form, so the ending stays the same.
I often forget when to use свой. Would it be wrong to say мой проездной here?
It wouldn't be strictly wrong, and people would understand you, but свой is heavily preferred here. Whenever the possessor is also the subject of the sentence (meaning 'I lost my own pass'), Russian uses the reflexive possessive свой to sound natural.