Коллекционер наткнулся на редкость.

Here наткнулся на means came across, ran into, or stumbled upon something by chance.

It often suggests an accidental discovery, not a planned search.

The verb can also have a literal meaning: to bump/stumble into something physically. But in this sentence it is clearly figurative: the collector unexpectedly found a rarity.

Because наткнуться на + accusative is the standard pattern.

So you learn it as a unit:

• наткнуться на что? = to come across / stumble upon something
• наткнуться на кого? = to run into someone

The preposition is not optional here. It is part of how the verb works in this meaning.

Because редкость is in the accusative singular, and for this noun the accusative looks the same as the nominative.

Why?

• редкость is a feminine noun ending in -ь
• it is inanimate
• with many inanimate nouns, the accusative = nominative

So:

• nominative: редкость
• accusative: редкость

Even though the form does not change, the function does: here it is the object of на after наткнулся.

It can mean either, depending on context.

In this sentence, it most naturally means a rare item / something rare, especially because the subject is Коллекционер. A collector would typically come across a rare object.

So although the literal noun is rarity, in English the idea is often closer to a rare find or a rare piece.

Because Russian past tense agrees with the subject in gender and number.

The subject is Коллекционер, which is masculine singular, so the verb is:

• masculine singular past: наткнулся

Compare:

• Коллекционер наткнулся... = masculine singular
• Коллекционерка наткнулась... = feminine singular
• Коллекционеры наткнулись... = plural

The -ся is the reflexive marker, but in this verb you should mostly treat it as part of the whole verb: наткнуться.

It does not mean the collector literally did something to himself in a simple, transparent way. In many Russian verbs, -ся is just part of the dictionary form and helps create a particular meaning.

So it is best to learn:

• наткнуться на что-то = to come across something

rather than trying to translate -ся word-for-word.

The dictionary form is наткнуться.

So the parts are:

• infinitive: наткнуться
• past masculine singular: наткнулся

This is a perfective verb, which matters for aspect.

Because the sentence describes a single completed event: the collector came across the rarity.

Russian aspect matters a lot:

• наткнуться = perfective, a completed discovery
• натыкаться / наталкиваться = imperfective, repeated or ongoing occurrences, or a more general process

So this sentence presents the event as one whole completed action.

For example:

• Коллекционер наткнулся на редкость. = He came across a rarity.
• Коллекционер часто натыкался на редкости на блошиных рынках. = He often came across rare items at flea markets.

Yes, Коллекционер наткнулся на редкость is a neutral, natural word order.

Russian word order is flexible, but changing it changes the focus or emphasis.

For example:

• Коллекционер наткнулся на редкость. = neutral
• На редкость наткнулся коллекционер. = marked/literary, emphasizing what he came across

One important caution: на редкость can also be an idiomatic expression meaning unusually in other sentences, such as:

• Он на редкость терпелив. = He is unusually patient.

So in this sentence, context and structure tell you that на редкость belongs with наткнулся на and means upon a rarity, not unusually.

The main stresses are:

• коллекционе́р
• наткну́лся
• ре́дкость

So roughly:

коллекционе́р наткну́лся на ре́дкость

Stress is important in Russian, so it is worth learning each word together with its stressed syllable.

Yes, but it is not exactly the same.

• нашёл редкость = found a rare item
• наткнулся на редкость = came across / stumbled upon a rare item

The second version more strongly suggests chance or accident.
So if the collector was deliberately searching and found it, нашёл may fit better. If he encountered it unexpectedly, наткнулся на is better.