Вчера в парке я услышал саксофон, а потом увидел молодого скрипача.

Because the sentence describes two completed events in sequence.

• услышать = to hear / to catch the sound of something at a particular moment
• увидеть = to see / to notice something at a particular moment

So услышал and увидел are perfective past forms, which fit a narrative like first this happened, then that happened.

If you used слышал or видел, it would sound more like an ongoing state, background information, or repeated experience.

В парке is in the prepositional case.

With в, Russian usually works like this:

• в + prepositional = location, in / at
• в + accusative = motion into, into

So:

• в парке = in the park
• в парк = into the park

Here the sentence gives a location, so в парке is correct.

It is the direct object, so it is in the accusative case. But for a masculine inanimate singular noun, the accusative looks the same as the nominative.

So:

• nominative: саксофон
• accusative: саксофон

That is why there is no visible ending change.

Also, услышал саксофон is natural Russian shorthand for heard the sound of a saxophone. It does not mean the instrument itself physically made contact with your ears; it means you heard it being played.

Because it is the direct object of увидел, so it must be in the accusative case.

For masculine animate nouns, the accusative looks like the genitive. That is why both the noun and adjective change:

• nominative: молодой скрипач
• accusative: молодого скрипача

This is a very important Russian pattern:

• masculine inanimate accusative often looks like nominative
• masculine animate accusative often looks like genitive

That is exactly why саксофон stays the same, but скрипач changes.

Yes. Скрипач is a masculine noun, so this wording refers to a male violinist.

If you wanted to say a young female violinist, a common form would be:

молодую скрипачку

So the sentence as written specifically points to a male violinist.

Because а joins two clauses:

• Вчера в парке я услышал саксофон
• а потом увидел молодого скрипача

In Russian, a comma is normally used before а in this kind of compound sentence.

Even though я is not repeated in the second clause, it is still understood there, so the comma remains correct.

Not exactly. А can mean but, but it also often marks a transition, contrast, or shift to the next part of the story.

Here а потом is best understood as something like:

• and then
• and after that

So it does not have a strong but meaning here. It simply moves the narration forward in a very natural Russian way.

Yes, it can sometimes be omitted, especially in conversation, if the subject is already clear.

But in the past tense, Russian verbs show gender and number, not person. So услышал by itself could mean:

• I heard (if the speaker is male)
• he heard

Because of that, я is often included to make the subject clear.

So я here is natural and helpful, not unnecessary.

The past-tense verbs change to the feminine form:

Вчера в парке я услышала саксофон, а потом увидела молодого скрипача.

Compare:

• masculine: услышал, увидел
• feminine: услышала, увидела

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

Russian word order is flexible, so this order is not the only possible one.

This version puts the setting first:

• Вчера = time
• в парке = place
• я = subject

That is very common in storytelling, because it sets the scene before the actions happen.

Other word orders are possible, for example:

• Я вчера в парке услышал саксофон...
• В парке вчера я услышал саксофон...

They are all grammatical, but the emphasis shifts slightly.

It usually does not. Russian has no articles like a and the.

So:

• саксофон could mean a saxophone or the saxophone
• молодого скрипача could mean a young violinist or the young violinist

The exact meaning depends on context, not on an article.

That is one reason why Russian learners have to rely more on the situation and surrounding sentences.