Друг показал находку.

Because of the case endings.

• друг is in the nominative form, which is the usual form for the subject.
• находку is in the accusative singular form of находка, so it is the direct object.

So the sentence structure is:

• друг = the friend
• показал = showed
• находку = the find / discovery

Even if the word order changed, the endings would still help you identify who did what.

Because it is a feminine noun used as a direct object, so it takes the accusative singular ending.

For most feminine nouns ending in -а, the accusative singular changes:

• -а → -у

So:

• находка = a find, discovery
• находку = the find, discovery (as the object)

This is a very common pattern:

• книга → книгу
• машина → машину
• находка → находку

In the past tense, Russian verbs agree with the subject in gender and number.

Here the subject is друг, which is a masculine singular noun, so the verb must be masculine singular past:

• показал = masculine singular
• показала = feminine singular
• показало = neuter singular
• показали = plural

So:

• Друг показал находку. = A male friend showed the find.
• Подруга показала находку. = A female friend showed the find.

The dictionary form is показать.

That is the infinitive, meaning to show.

Here is the relationship:

• показать = to show
• показал = showed / has shown

More precisely, показать is a perfective verb, so it refers to a completed act of showing.

Показал comes from the perfective verb показать, which presents the action as completed.

So Друг показал находку means the friend showed it successfully / as a complete action.

If you used the imperfective показывал, the meaning would be different:

• Друг показал находку. = The friend showed the find.
  completed event
• Друг показывал находку. = The friend was showing / used to show the find.
  process, repetition, or background action

English often does not force this distinction as clearly as Russian does, so this is an important thing to notice.

Russian has no articles.

So друг can mean:

• a friend
• the friend

And находку can mean:

• a find
• the find

The exact meaning depends on context.

English speakers often want a one-to-one equivalent for a or the, but Russian usually leaves that unstated unless another word makes it clearer.

Usually, yes: друг normally refers to a male friend.

The usual feminine form is:

• подруга = female friend

So if the person who showed the find was female, you would usually say:

• Подруга показала находку.

However, in some contexts, especially older or more general phrasing, masculine forms can sometimes be used more broadly, but for everyday learning it is best to understand:

• друг = male friend
• подруга = female friend

Yes. Russian word order is more flexible than English word order because case endings carry a lot of grammatical information.

For example:

• Друг показал находку.
• Находку показал друг.
• Показал друг находку.

These all can mean roughly the same basic thing, but the focus changes.

• Друг показал находку.
  neutral: the friend showed the find.
• Находку показал друг.
  emphasizes the find or contrasts who showed it.
• Показал друг находку.
  sounds more marked or literary, depending on context.

So the original sentence is the most neutral and natural basic order.

The stress is:

• друг
• показа́л
• нахо́дку

A simple pronunciation guide:

• друг ≈ drook
• показа́л ≈ puh-kah-ZAHL
• нахо́дку ≈ nah-HOT-koo

A couple of useful notes:

• Final г in друг is pronounced like k at the end of the word.
• Unstressed о is often pronounced more like a in connected speech.

Находка usually means a find, a discovery, or something that has been found.

Depending on context, it could be:

• an object someone found
• an archaeological find
• an interesting discovery
• even, in some contexts, a figurative great find or real gem

So in this sentence, находку is the thing the friend showed.

Yes, depending on context.

Russian does not have a separate form exactly matching the English present perfect in the way English does. A past-tense perfective verb like показал can often correspond to:

• showed
• has shown

Which translation sounds best depends on the surrounding context.

So Друг показал находку could be translated naturally as:

• The friend showed the find
• The friend has shown the find

In many cases, plain showed is the most natural default translation.

It is a perfectly complete sentence on its own.

Russian often allows short, compact sentences like this:

• subject
• past-tense verb
• direct object

So Друг показал находку. is a normal, complete statement.

That said, because Russian has no articles and often leaves some things to context, an English speaker may feel it is a little more open-ended than an English sentence. For example, context would tell you:

• which friend
• which find
• when exactly it happened
• why it mattered

But grammatically, the sentence is complete.