Learn Hardcore Russian: Мой младший брат любит играть в парке. - My younger brother loves to play in the park.

младший – younger in a family/seniority sense: младший брат, младшая сестра, младший сын.
молодой – young in terms of age generally: молодой человек (a young man), молодая женщина.
маленький – small/little (size or sometimes age, but emotionally/affectionately):
- маленький брат – “little brother” (can imply size or age, sounds more affectionate/childish).

In your sentence, младший брат clearly means “my younger brother (not the older one).”

Why is there no он (he) before любит?

Russian often omits subject pronouns when the subject is clear from the noun or from the verb ending.

Here, the subject is already expressed:

Мой младший брат – subject (my younger brother)
любит – 3rd person singular verb form, which matches он / брат.

Saying Мой младший брат он любит играть в парке would sound wrong or at least very unnatural. The noun брат itself is enough; no pronoun is needed.

Why is it любит and not любить?

Любить is the infinitive (“to love, to like”).
In a sentence with a subject, you normally need a conjugated (finite) form:

он любит – he loves / he likes
я люблю – I like
мы любим – we like

Since the subject is мой младший брат (3rd person singular), the verb must be любит, not the infinitive любить.

The infinitive appears later in the sentence: любит играть – “likes to play.”

Why do we say любит играть instead of just играет?

The nuances are different:

Мой младший брат играет в парке.
= My younger brother is playing / plays in the park (describes an action, either right now or habitually).
Мой младший брат любит играть в парке.
= My younger brother likes playing in the park (describes a preference / enjoyment, not just the fact that he plays).

So любит играть = “likes to play / likes playing,” a very typical Russian structure:
любить + infinitive to express liking an activity:

Она любит читать. – She likes to read.
Мы любим путешествовать. – We like to travel.

Why is the verb играть in the imperfective aspect? Could we use a perfective form like сыграть here?

In Russian, to express a general habit or preference, you almost always use the imperfective aspect.

играть (imperfective) – to play (process, repeated, general)
сыграть (perfective) – to play once / to complete a play of something (e.g. to play a game, a piece of music, a match, to the end)

In любит играть, we talk about what he enjoys in general, not about a single finished event, so imperfective играть is correct.

любит сыграть is possible, but it usually implies a single action with a bit of nuance like “He likes to (occasionally) have a game / play a round,” often with context (cards, chess, etc.). It’s much less neutral than любит играть.

Why is it в парке and not на парке?

Russian uses в and на with locations in ways that often don’t match English in / on exactly.

For most closed or defined places like buildings, rooms, and many public areas, you use в:

в парке – in the park
в школе – at school / in school
в магазине – at the shop / in the store
в городе – in the city

на is used with some open areas, surfaces, and certain fixed expressions:

на улице – in the street / outside
на площади – in the square
на крыше – on the roof
на стадионе – at the stadium

With парк, the standard is в парке (“in the park”). на парке would be incorrect in normal Russian.

Why does парк change to парке?

This is the prepositional case (also called “locative” in older descriptions), which is used after в and на when they mean location (“in / at / on where?”).

The basic noun is:

парк – nominative singular (dictionary form).

After в with the meaning in a place, masculine nouns ending in a consonant usually take -е:

в парке – in the park
в доме – in the house (дом → доме)
в лесу (irregular; лес → лесу, but often learners see лесе too in older materials)
в магазине – in the shop (магазин → магазине)

So парк → парке is just the regular prepositional singular ending: -е.

Can I change the word order, for example: В парке мой младший брат любит играть?

Yes. Russian word order is more flexible than English, though there are natural preferences.

All of these are grammatically correct:

Мой младший брат любит играть в парке.
– neutral, very natural; focus on the fact that he likes to play (and the place is just part of the information).
В парке мой младший брат любит играть.
– puts В парке at the front, emphasizing the location: As for in the park, my younger brother likes to play there.
Мой младший брат в парке любит играть.
– possible, but sounds a bit more stylized; might contrast “in the park” with other places (e.g. “At home he doesn’t, but in the park he likes to play”).

For a beginner, the original word order Subject – Verb – Infinitive – Place is the safest and most natural:
Мой младший брат любит играть в парке.

Could I omit мой and just say Младший брат любит играть в парке?

You can, but the meaning and naturalness change.

Мой младший брат любит играть в парке.
– clearly means my younger brother.
Младший брат любит играть в парке.
– literally “(The) younger brother likes to play in the park.”
Without context, this sounds a bit incomplete: whose younger brother? It might work if it’s obvious from context (for example, you just mentioned an older and a younger brother in a family).

In Russian, possessive pronouns with close family members are often omitted when the owner is absolutely obvious:

Pointing to your mother in front of you: Это мама. (obviously “this is my mother”)
Talking about your own parents: Папа пришёл. (Dad has come.)

In a stand‑alone sentence like this, Мой младший брат… is more natural if you mean my younger brother and the context doesn’t already make that obvious.