После прогулки у озера я вытряхнул песок из кроссовок.

Because после requires the genitive case.

• прогулка = nominative singular
• прогулки = genitive singular

So после прогулки literally means after a walk.

This is a very common pattern in Russian:

• после урока = after the lesson
• после работы = after work
• после дождя = after the rain

So the form прогулки is there because of the preposition после.

Here у means something like by, near, or beside.

So:

• озеро = lake
• у озера = by the lake / near the lake

The noun is озера because у also requires the genitive case.

• озеро → nominative
• озера → genitive singular

This use of у is very common for location:

• у дома = by the house
• у реки = by the river
• у моря = by the sea

So прогулка у озера means a walk by the lake.

Вытряхнул comes from вытряхнуть, which means to shake out.

So in this sentence, it means the speaker shook the sand out of the sneakers.

It is made from the verb трясти / тряхнуть related to shaking, with the prefix вы-, which often adds the idea of out.

So:

• тряхнуть = to give something a shake
• вытряхнуть = to shake something out

This verb fits very naturally with sand, dust, crumbs, stones, water, etc.:

• вытряхнуть песок из обуви = shake the sand out of the shoes
• вытряхнуть крошки из пакета = shake the crumbs out of the bag

Yes, вытряхнул is perfective past tense.

The infinitive is вытряхнуть. Perfective verbs usually describe a completed action. Here, the speaker finished shaking the sand out.

So the sentence presents the action as a single completed event:

• я вытряхнул песок = I shook the sand out

The imperfective partner is usually вытряхивал / вытряхивать, which would suggest process, repetition, or ongoing action:

• я вытряхивал песок из кроссовок = I was shaking / used to shake the sand out of my sneakers

In this sentence, the completed action is the natural choice.

Because песок is the direct object of вытряхнул, so it is in the accusative case.

For masculine inanimate nouns, the accusative singular is often the same as the nominative singular:

• песок = nominative
• песок = accusative

So:

• я вытряхнул что? → песок

Compare with another masculine inanimate noun:

• я вижу стол = I see a table

But with animate masculine nouns, accusative usually matches genitive:

• я вижу брата = I see my brother

So песок stays the same because it is masculine, singular, and inanimate.

Из requires the genitive case, so кроссовок is genitive plural.

The basic form is:

• кроссовки = sneakers / trainers

This noun is normally used in the plural, because it refers to a pair of shoes.

So:

• кроссовки = nominative plural
• из кроссовок = out of the sneakers = genitive plural

The pattern is:

• вытряхнуть что из чего
• shake something out of something

Examples:

• вытряхнуть воду из бутылки = shake the water out of the bottle
• вытряхнуть камни из кармана = shake the stones out of the pocket
• вытряхнуть песок из кроссовок = shake the sand out of the sneakers

It is a normal genitive plural form for this noun.

The dictionary form is кроссовки. Its genitive plural is кроссовок.

Russian genitive plural forms can be unpredictable, and many nouns do not simply add an obvious ending. For this word, the correct form is:

• кроссовки → кроссовок

Similar examples:

• ботинки → ботинок
• носки → носков
• туфли → туфель

So yes, кроссовок may look unusual at first, but it is the standard form.

Yes, я could be omitted in many contexts.

Russian often leaves out subject pronouns when the meaning is clear from context. So both of these are possible:

• После прогулки у озера я вытряхнул песок из кроссовок.
• После прогулки у озера вытряхнул песок из кроссовок.

However, there is an important detail: in the past tense, Russian verbs show gender and number, but not person very clearly in the way present tense does. So adding я can make the sentence clearer and more natural, especially out of context.

Also, including я can add a bit of emphasis: I shook the sand out.

Yes. It is the past tense masculine singular form.

Russian past tense forms agree with the subject in gender and number:

• я вытряхнул = I shook out... (male speaker)
• я вытряхнула = I shook out... (female speaker)
• мы вытряхнули = we shook out...

So this sentence suggests that the speaker is male.

That is a very common thing for English speakers to notice, because English past tense does not show the speaker’s gender.

Yes, Russian word order is flexible.

This sentence is perfectly natural:

• После прогулки у озера я вытряхнул песок из кроссовок.

It starts with the time/context phrase После прогулки у озера and then gives the main action.

Other orders are also possible, depending on emphasis:

• Я вытряхнул песок из кроссовок после прогулки у озера.
• Песок из кроссовок я вытряхнул после прогулки у озера.

The original version sounds natural because it sets the scene first: After a walk by the lake...

That kind of ordering is very common in Russian narrative style.

The most natural interpretation is after a walk by the lake.

The phrase прогулка у озера is a noun phrase:

• прогулка = walk
• у озера = by the lake

So together it means a walk by the lake.

This whole phrase then goes into the genitive after после:

• после прогулки у озера = after a walk by the lake

In English there are several ways to translate it, but grammatically in Russian, у озера modifies прогулки.

Because Russian has no articles like a or the.

Russian usually leaves definiteness to context. So:

• песок can mean sand, the sand, or sometimes some sand
• у озера can mean by a lake or by the lake
• из кроссовок can mean out of the sneakers / my sneakers / the sneakers, depending on context

In this sentence, context makes it clear that it means something like the sand and the/my sneakers.

This is one of the biggest differences from English: Russian often does not explicitly mark whether something is definite or indefinite.

Кроссовки usually means sneakers, trainers, or athletic shoes.

It is a fairly specific word, not just any shoes in general.

Compare:

• обувь = footwear
• туфли = dress shoes / women’s shoes, depending on context
• ботинки = boots / sturdy shoes
• кроссовки = sneakers / trainers

So in this sentence, the image is specifically casual sports shoes with sand inside them.

A useful way to break it down is:

• После прогулки у озера = after a walk by the lake
• я = I
• вытряхнул = shook out
• песок = sand
• из кроссовок = out of the sneakers

So the structure is:

time/context phrase + subject + verb + direct object + из + source

More abstractly:

После + genitive
у + genitive
verb + accusative + из + genitive

This sentence is a good example of how Russian uses cases instead of articles or rigid word order to show relationships between words.