Друг предложил обменяться находками.

Предложил is the past tense of предложить (to suggest, to offer).

Its form tells us:

• the action happened in the past;
• the subject is singular;
• the subject is masculine.

So Друг предложил... means that a male friend made the suggestion. If the subject were female, it would be подруга предложила or друг предложила only in a very unusual context.

In Russian, предложить can be followed by an infinitive, just like English to suggest doing something.

So:

• Друг предложил обменяться находками = A friend suggested exchanging finds

This structure is very common:

• Он предложил пойти домой = He suggested going home
• Она предложила подождать = She suggested waiting

Russian can also use a чтобы clause, but the infinitive is very natural here.

Обменяться means to exchange with each other.

It is not just to exchange something in a general sense. It has a mutual meaning: two or more people give something to one another.

So here it means something like:

• to exchange our finds
• to swap finds with each other

That mutual idea is important.

The -ся here gives the verb a reciprocal meaning: with each other.

Compare:

• обменять = to exchange/swap something
• обменяться = to exchange things with each other

So -ся does not simply mean oneself here. In this sentence, it shows a mutual action between people.

Because обменяться normally takes the thing exchanged in the instrumental case.

So:

• обменяться идеями = to exchange ideas
• обменяться номерами = to exchange phone numbers
• обменяться находками = to exchange finds

This is something you largely have to learn as part of the verb’s pattern:

• обменяться + instrumental

Находками is the instrumental plural of находка.

The plural suggests finds, discoveries, or things found, rather than just one item.

Singular would be:

• находкой = with a find / a discovery

But here the idea is that they are exchanging multiple finds, so plural is more natural.

Находка literally means a find — something that has been found.

Depending on context, it can mean:

• a physical object someone found;
• a useful discovery;
• an interesting find or observation.

So находками could mean:

• objects they found;
• discoveries they made;
• interesting things they came across.

The exact shade depends on context.

Yes, the sentence leaves that implicit.

Обменяться already implies at least two participants. The sentence says:

• A friend suggested exchanging finds

Russian does not need to spell out the second participant if it is obvious from context. Usually it means:

• the friend and the speaker;
• or the friend and some other person already being discussed.

So the missing idea is something like we exchange with each other.

Both verbs are perfective because the sentence presents the actions as complete, whole events.

• предложил = he made the suggestion once, as a completed act
• обменяться = to carry out one complete exchange

Compare:

• предлагал = he was suggesting / used to suggest
• обмениваться = to exchange repeatedly or as an ongoing process

So this sentence sounds like a single concrete suggestion about a single exchange.

Yes, but it would change the meaning.

• обменяться находками suggests one completed exchange
• обмениваться находками suggests repeated or ongoing exchange, like to keep sharing finds

For example:

• Друг предложил обменяться находками = one specific swap
• Друг предложил обмениваться находками = he suggested that we exchange finds regularly / keep sharing them

Russian word order is flexible, so it can change, but the neutral order here is very natural:

• Друг предложил обменяться находками.

This simply presents:

1. the subject,
2. the action,
3. the rest of the idea.

Other orders are possible for emphasis:

• Обменяться находками друг предложил.
• Находками друг предложил обменяться.

But those sound more marked and would usually depend on context.

Russian has no articles, so друг can mean either a friend or the friend, depending on context.

So this sentence could mean:

• A friend suggested exchanging finds or
• The friend suggested exchanging finds

You decide from the broader situation, not from the noun form itself.

The main stress is:

Друг предложи́л обменя́ться нахо́дками.

A rough pronunciation guide:

• друг = drook
• предложи́л = pryeed-la-ZHEEL
• обменя́ться = ab-myee-NYAT-sa
• нахо́дками = na-HOT-ka-mi

The stressed syllables are:

• -жи́л
• -ня́ть-
• хо́д

That stress is important, especially in предложи́л and обменя́ться.