Друг узнал комбинацию и открыл сейф.

Because друг is the subject of the sentence, so it is in the nominative case.

• друг = friend
• nominative singular = друг

In this sentence, друг is the person doing both actions:

• узнал = learned/found out
• открыл = opened

So друг stays in the subject form.

Because it is the direct object of узнал, so it takes the accusative case.

The noun комбинация is feminine, and feminine nouns ending in -я usually change like this in the accusative singular:

• nominative: комбинация
• accusative: комбинацию

So:

• узнал комбинацию = learned/found out the combination

This is a very common feminine accusative pattern.

Because сейф is a masculine inanimate noun, and in Russian, masculine inanimate nouns usually have the same form in the accusative as in the nominative.

So:

• nominative: сейф
• accusative: сейф

That is why you get:

• открыл сейф = opened the safe

This is different from animate masculine nouns, which often change differently in the accusative.

They are in the past tense, masculine singular form.

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

• masculine singular: usually ends in -л
• feminine singular: usually ends in -ла
• neuter singular: usually ends in -ло
• plural: usually ends in -ли

Since друг is one male friend, the verbs are:

• узнал
• открыл

If the subject were female, you would get:

• подруга узнала комбинацию и открыла сейф

Russian has no articles.

So друг can mean:

• a friend
• the friend

And сейф can mean:

• a safe
• the safe

You understand which one is meant from the context.

That is normal in Russian. Learners often want to insert something like the, but Russian simply does not use articles.

Because узнал means found out / learned / came to know, while знал means knew.

This is an aspect and meaning difference:

• знал комбинацию = knew the combination
• узнал комбинацию = found out the combination / learned the combination

In this sentence, the idea is that the friend first obtained that knowledge, and then opened the safe. So узнал fits much better.

Both are perfective verbs:

• узнать → узнал
• открыть → открыл

Perfective verbs present actions as completed events. That works well here because the sentence describes a sequence of completed actions:

1. he found out the combination
2. he opened the safe

If you used imperfective verbs, the meaning would shift more toward process, repetition, or background description.

Because Russian does not need to repeat the subject when it is already clear.

In English, you normally say:

• The friend learned the combination and opened the safe.

You do not repeat he, and Russian works the same way here.

So:

• Друг узнал комбинацию и открыл сейф.

means the same subject, друг, did both actions.

Because и joins two verbs with the same subject inside one simple sentence.

Structure:

• Друг = subject
• узнал and открыл = two coordinated predicates

So no comma is needed:

• Друг узнал комбинацию и открыл сейф.

A comma would usually appear only if there were separate clauses with their own structure, or for other punctuation reasons.

Yes, Russian word order is relatively flexible.

The most neutral order here is:

• Друг узнал комбинацию и открыл сейф.

But other orders are possible, depending on emphasis:

• Комбинацию друг узнал и открыл сейф.
• Сейф друг открыл, узнав комбинацию.

These alternatives are less neutral and would usually need a particular context. For a learner, the original sentence is the most straightforward, standard order.

Yes, узнать can sometimes mean recognize, depending on context.

For example:

• Я узнал его = I recognized him

But with комбинацию, the more natural meaning is learned / found out the combination, not recognized the combination.

So the object often helps determine the meaning.

It can be used, and it is understandable, but in modern everyday Russian people also very often say код for a code or combination.

So depending on context, you might hear:

• узнал комбинацию
• узнал код
• ввёл код
• подобрал код

All of these can be natural in discussions about locks and safes.
Still, комбинацию is perfectly understandable, especially when you want the specific idea of a combination lock.

From two things:

1. друг is the masculine noun for male friend
2. the past tense verbs are masculine: узнал, открыл

If the friend were female, you would normally use:

• подруга узнала комбинацию и открыла сейф.

So Russian gives you gender information more directly than English does in this sentence.

They are:

• узнать = to find out, to learn
• открыть = to open

The sentence uses their past masculine singular forms:

• узнать → узнал
• открыть → открыл

Learning to connect dictionary infinitives to past forms is very important in Russian.

The stress is:

• друг
• узна́л
• комбина́цию
• и
• откры́л
• се́йф

So the whole sentence is pronounced roughly:

• друг узна́л комбина́цию и откры́л се́йф

The stressed syllables matter a lot in Russian, because stress is not always predictable.

Because Russian past tense works differently from English present tense.

In the past tense, Russian verbs usually show:

• gender in the singular
• number

They do not show person the way present/future forms do.

So узнал by itself can mean:

• I found out
• you found out
• he found out

The subject or context tells you which one it is.

Here, друг makes it clear that it means the friend found out.

Not directly from the Russian wording alone.

• открыл сейф simply means opened the safe
• узнал комбинацию gives the reason or prior step

So the full idea may imply success, and in natural English you might sometimes say managed to open the safe, but Russian does not explicitly use a special verb for managed to here.

The sentence is more literally:

• The friend found out the combination and opened the safe.