Bilet stracił ważność.

Because bilet is a masculine singular noun in Polish.

In the past tense, Polish verbs agree with the subject in gender and number:

• stracił = masculine singular
• straciła = feminine singular
• straciło = neuter singular
• straciły / stracili = plural

So:

• Bilet stracił ważność. = The ticket lost its validity / expired.
• Karta straciła ważność. = The card expired.

Ważność is a noun meaning validity.

It comes from the adjective ważny, which means valid, important, or serious, depending on context. In this sentence, it means valid in the sense of something officially being in force.

So:

• ważny bilet = a valid ticket
• ważność biletu = the validity of the ticket

In Bilet stracił ważność, the idea is literally The ticket lost validity.

Polish often uses stracić ważność as a natural expression meaning:

• to expire
• to cease to be valid
• to lose validity

It is very common for things like:

• tickets
• documents
• passports
• contracts
• cards
• permits

A different but also correct way to express a similar idea is:

• Bilet jest nieważny. = The ticket is invalid / not valid.

The difference is roughly:

• Bilet stracił ważność. focuses on the change: it became invalid.
• Bilet jest nieważny. focuses on the current state: it is invalid.

Yes, very much so.

Stracić ważność is a standard collocation in Polish. It is used especially in official, legal, and practical contexts.

Common examples:

• Paszport stracił ważność. = The passport expired.
• Umowa straciła ważność. = The contract ceased to be valid.
• Dowód osobisty stracił ważność. = The ID card expired.

So even though it literally means lost validity, you should learn it as a normal set phrase.

Ważność is in the accusative singular here, because it is the direct object of the verb stracić (to lose).

The structure is:

• bilet = subject, nominative
• stracił = verb
• ważność = object, accusative

For this noun, the nominative and accusative forms look the same:

• nominative: ważność
• accusative: ważność

So you do not see a visible ending change, but grammatically it is functioning as the object.

Stracił comes from stracić, which is a perfective verb.

In Polish, aspect is very important:

• stracić = perfective, focusing on the completed result
• tracić = imperfective, focusing on the process or repeated action

So:

• Bilet stracił ważność. = The ticket expired / has expired.
  Completed event.

Compare:

• Bilet traci ważność jutro. = The ticket expires tomorrow.
• Ten dokument szybko traci ważność. = This document quickly loses validity.

In your sentence, the perfective verb fits because the ticket’s validity has already ended.

Yes.

Polish past tense often covers meanings that English expresses with either:

• simple past: The ticket expired
• present perfect: The ticket has expired

Which English version sounds best depends on context.

So Bilet stracił ważność can naturally correspond to either of these in English.

It can be similar, but it is not always the most natural equivalent.

Wygasnąć means something like:

• to expire
• to lapse
• to run out
• to cease to be in force

For official validity, stracić ważność is often the clearest and most standard phrase.

Examples:

• Umowa wygasła. = The contract expired / lapsed.
• Licencja wygasła. = The license expired.

With bilet, stracił ważność sounds especially straightforward and standard.

Yes, you can.

Compare the two:

• Bilet stracił ważność. = The ticket has expired / lost validity.
• Bilet jest nieważny. = The ticket is invalid.

The first emphasizes what happened.
The second emphasizes the current situation.

Also, nieważny can sometimes suggest not only that it expired, but more generally that it is not acceptable or not legally valid for some reason.

So if you specifically mean it used to be valid, but now it isn’t, stracił ważność is often the better choice.

Because Polish often does not need to say its when the meaning is obvious from context.

In English, we say:

• The ticket lost its validity.

In Polish, it is natural to say simply:

• Bilet stracił ważność.

The relationship is understood automatically: the ticket lost its own validity.

If Polish speakers added a possessive word here, it would usually sound unnecessary.

A rough guide:

• stracił ≈ STRAH-chiw
• ważność ≈ VAZH-noshch

A few useful points:

• rz / ż sounds like the s in measure
• ci before a vowel often sounds like a soft ch / ć-type sound
• ś, ć, ń, ź are soft consonants
• ą and ę are nasal vowels, but there is no nasal vowel in this sentence
• -ść / -ść / -ność endings often sound compressed to English ears

If you want a more careful approximation:

• stracił = STRA-ciw, with a soft consonant in the middle
• ważność = VAZH-noshch, with the final cluster pronounced very tightly

A natural way is:

• Bilet straci ważność. = The ticket will expire / lose validity.

Because stracić is perfective, its non-past form usually has future meaning:

• straci = will lose

You can also say:

• Bilet straci ważność jutro. = The ticket will expire tomorrow.

You would say:

• Bilet nie stracił ważności. = The ticket did not lose validity / has not expired.

Notice something important: when a direct object is negated in Polish, it often changes from accusative to genitive.

So:

• positive: stracił ważność
• negative: nie stracił ważności

That change is very common in Polish and is worth learning early.

Because Polish often uses the genitive instead of the accusative with negated verbs.

So:

• stracił ważność
  direct object in the accusative

but:

• nie stracił ważności
  object changes to the genitive because of negation

This is a common Polish pattern, although modern usage can vary in some contexts. In a sentence like this, ważności is the expected standard form.