Learn Hardcore Russian: В солнечный день, наоборот, я езжу в парк на велосипеде и долго там катаюсь. - On a sunny day, on the contrary, I go to the park by bicycle and ride there for a long time.

Grammatically, день is masculine; its Accusative singular is день, and the adjective солнечный agrees and becomes солнечный (Accusative masculine inanimate looks like Nominative).

в солнечном дне is grammatically possible but sounds strange here; it would mean something like “in the sunny day” as if the day were a physical space you are inside. For talking about when something happens, Russian prefers в солнечный день with the Accusative.

What exactly does наоборот add here, and is it necessary?

Наоборот means “on the contrary / the opposite”. It signals contrast with something mentioned just before, for example:

В дождливый день я сижу дома. В солнечный день, наоборот, я езжу в парк…
On a rainy day I stay at home. On a sunny day, by contrast, I go to the park…

So in this sentence, наоборот tells us:

in other (contrasting) weather conditions, the speaker does something different (probably stays home),
but on a sunny day, they instead go to the park by bike.

Is it necessary?

Grammatically: no; you can say В солнечный день я езжу в парк на велосипеде…
Stylistically: it’s useful if this sentence is contrasting with a previous one. Without наоборот, the idea of “by contrast” disappears.

Why are there commas around наоборот: В солнечный день, наоборот, я езжу…?

Here наоборот is used as a parenthetical word (вводное слово). Russian punctuation rules say that such words are usually separated by commas.

So the structure is:

В солнечный день, – introductory adverbial phrase (time)
наоборот, – parenthetical “on the contrary”
я езжу… – main clause

That’s why we see:

В солнечный день, наоборот, я езжу в парк…

You could also move or omit it:

В солнечный день я, наоборот, езжу в парк… – emphasis on “I, on the contrary, go…”
В солнечный день я езжу в парк… – no idea of contrast.

What is the difference between езжу and еду, and why is езжу used here?

Both come from the verb ехать / ездить – “to go (by transport), to ride, to travel”, but:

ехать – uni‑directional, one trip in one direction, usually right now / once
- Я еду в парк. – I am going (riding) to the park (right now).
ездить – multi‑directional, repeated, habitual, back‑and‑forth, or “in general”
- Я езжу в парк. – I (regularly) go / ride to the park.

In the sentence:

…я езжу в парк на велосипеде…

the point is a habit – “On a sunny day I (typically) go to the park by bike.”
That’s why the multi‑directional ездить → езжу is used, not еду.

Why is it езжу в парк, not something like езжу в парке?

With the preposition в, Russian distinguishes:

в + Accusative = movement to / into a place (direction)
в + Prepositional = location in a place (where)

So:

я езжу в парк – I go (ride) *to the park (destination; *в парк = Accusative)
я катаюсь в парке – I ride *in the park (location; *в парке = Prepositional)

In the sentence, езжу в парк talks about going to the park; later, там катаюсь describes what happens there once the person has arrived.

Why is it на велосипеде and not велосипедом?

Both are grammatically possible, but на велосипеде is the standard, most natural way to say “by bike / on a bicycle”:

на велосипеде – literally “on the bicycle”; very common for types of transport:
- на автобусе – by bus
- на поезде – by train
- на машине – by car
- на велосипеде – by bike

велосипедом (Instrumental) can also appear, but usually has a slightly different nuance, more like “with a bicycle / using a bicycle”, or in more specific stylistic contexts. For simple “I go to the park by bike”, на велосипеде is the natural choice:

…я езжу в парк на велосипеде… – I ride my bike to the park.

What is the nuance of езжу на велосипеде vs катаюсь на велосипеде?

Both involve a bike, but they focus on different things:

ездить на велосипеде
- more about using the bike as transport, going somewhere;
- e.g. Я езжу на велосипеде в школу. – I go to school by bike.
кататься на велосипеде
- more about riding around for fun / recreation, not about reaching a destination;
- e.g. Я катаюсь на велосипеде в парке. – I ride my bike around in the park (for fun).

In the sentence:

…я езжу в парк на велосипеде и долго там катаюсь.

езжу в парк на велосипеде – I use the bike to get to the park.
долго там катаюсь – once there, I ride around for a long time for pleasure.

What does катаюсь mean, and why does it end with -сь?

Катаюсь is the 1st person singular present of кататься.

кататься = to ride / slide / roll around (usually for fun)
- кататься на велосипеде – ride a bike (for fun)
- кататься на лыжах – ski
- кататься на коньках – skate
- кататься на карусели – ride a carousel

The ending -сь (or -ся) marks a reflexive / middle verb form in Russian. It doesn’t mean “myself” here; it’s just part of how this particular verb is formed and used.

So:

я катаюсь – I ride (around)
ты катаешься – you ride (around)
он катается – he rides (around)

Why is я not repeated before катаюсь?

Russian doesn’t require subject pronouns to be repeated when it’s clear who the subject is. Verbs are inflected and already show person and number.

The full logical structure is:

…я езжу в парк на велосипеде и (я) долго там катаюсь.

The second я is usually dropped because:

we already know the subject is я,
катаюсь is 1st person singular, so the subject is clear from the verb ending.

Repeating я here (…и я долго там катаюсь) is possible but would add extra emphasis to “I”, which is not needed in a neutral sentence.

What does долго mean here, and can the word order долго там катаюсь be changed?

Долго means “for a long time” (duration).

долго катаюсь – I ride for a long time.

Там means “there”, referring to the park.

The phrase долго там катаюсь literally is:

долго – for a long time
там – there
катаюсь – (I) ride (around)

Word order in Russian is relatively flexible. You could say:

там долго катаюсь
катаюсь там долго
долго катаюсь там

All are grammatically correct; the differences are mainly in rhythm and minor emphasis. The original долго там катаюсь is a neutral, natural-sounding order.