Learn Hardcore Russian: Моя сестра любит создавать уютную атмосферу дома. - My sister likes to create a cozy atmosphere at home.

Она любит создавать… = She likes to create / She likes creating…
Not Она любит создаёт… – that’s ungrammatical (you can’t follow любит with a finite verb).

If you drop любить, then you can say:

Она создаёт уютную атмосферу дома. – She creates a cozy atmosphere at home.
Here создаёт is just the main verb in the present tense, not an infinitive after любить.

Why is it создавать and not создать after любит?

Создавать and создать are an imperfective / perfective verb pair:

создавать – imperfective: to be creating, to create in general, to do something as a process or habit.
создать – perfective: to create (once), to complete the act, result‑focused.

After verbs of liking (любить, нравиться) you almost always use the imperfective infinitive because you’re talking about an activity in general, not one completed action:

Она любит создавать уютную атмосферу.
She likes creating a cozy atmosphere (in general, regularly).

Using любить создать would sound wrong and unnatural; любить is about general preference, which ties naturally to the imperfective aspect.

What case is уютную атмосферу and how do the endings work?

Уютную атмосферу is in the accusative singular, because it’s the direct object – the thing being created.

атмосфера (atmosphere) – feminine noun.
- Nominative: атмосфера
- Accusative: атмосферу (‑а → ‑у for feminine animate/inanimate with this pattern)
уютный (cozy) – adjective.
- Feminine nominative: уютная
- Feminine accusative: уютную (‑ая → ‑ую)

Adjectives must agree with the noun in:

gender: feminine,
number: singular,
case: accusative.

So we get:

уютную атмосферу = a cozy atmosphere (as a direct object).

Compare:

Nominative: уютная атмосфера (A cozy atmosphere is important.)
Accusative: уютную атмосферу (She creates a cozy atmosphere.)

Why isn’t it уютная атмосфера, like the dictionary form?

Уютная атмосфера is the nominative form (used for the subject of the sentence).
But here “atmosphere” is not the subject; it is the object of the verb “to create”.

Subject: Кто? Что? (Who? What?) – nominative.
Direct object: Кого? Что? (Whom? What?) – accusative.

Your sentence answers:

Who creates? → Моя сестра (subject, nominative)
Creates what? → уютную атмосферу (object, accusative)

So both the noun and its adjective must change to accusative:

уютная атмосфера → уютную атмосферу.

What exactly does уютный mean, and are there related words I should know?

Уютный roughly corresponds to cozy / homely / comfortable in the sense of a pleasant, warm, inviting atmosphere.

Useful related words:

уют – coziness, comfort (as a noun, the quality itself)
- Мне нравится уют в этом доме. – I like the coziness in this house.
уютно – cozy / comfortable (as an adverb or predicative)
- Здесь очень уютно. – It’s very cozy here.
сделать комнату уютной – to make a room cozy.

So создавать уютную атмосферу literally means “to create a cozy atmosphere,” i.e., to create a feeling of comfort and homeliness.

What form is дома, and how is it different from дом, в доме, and домой?

Дома here means “at home” and functions like an adverbial form.

Forms of дом (house, home):

дом – nominative: a house / home
- Это мой дом. – This is my house.
в доме – “in the house” (prepositional case with в)
- Мы в доме. – We are in the house.
дома – “at home” (genitive/locative form used adverbially)
- Я дома. – I am at home.
- создавать уютную атмосферу дома – to create a cozy atmosphere at home.
домой – “(to) home” (direction)
- Я иду домой. – I am going home.

So in your sentence, дома tells us where she creates this atmosphere: at home.

Could the word order be different, like Моя сестра дома любит создавать уютную атмосферу?

Yes, Russian word order is more flexible than English.

Possible variants (all grammatically correct):

Моя сестра любит создавать уютную атмосферу дома.
Моя сестра дома любит создавать уютную атмосферу.
Дома моя сестра любит создавать уютную атмосферу.

The basic meaning stays the same, but nuances of emphasis can shift:

Placing дома earlier (e.g. Дома моя сестра любит…) can slightly emphasize “at home, my sister likes…”, contrasting with other places.
Keeping the original order is neutral and very natural.

Importantly:

The subject моя сестра normally stays near the start.
The verb complex любит создавать stays together.
The object уютную атмосферу and the adverbial дома can move a bit for emphasis.

Why do we use любит, not нравится, to say “likes”?

Both любить and нравиться can translate as “to like”, but they work differently:

любить (to love / to like):
- Regular transitive verb.
- Pattern: Кто? + любит + что?
  - Моя сестра любит создавать уютную атмосферу.
нравиться (to be pleasing to someone):
- “Backward” structure.
- Pattern: Кому? + нравится + что?
  - Моей сестре нравится создавать уютную атмосферу.
    (Literally: To my sister it is pleasing to create…)

In practice:

любить + infinitive is very common for hobbies and preferences:
Она любит читать. – She likes reading.
нравиться can be a bit more neutral and impersonal:
Ей нравится читать. – Reading is pleasing to her.

Your sentence uses the straightforward любить + infinitive pattern.

How would the sentence change if it were my brother instead of my sister?

You need to change the possessive pronoun and the noun to masculine:

Моя сестра любит создавать уютную атмосферу дома.
→ Мой брат любит создавать уютную атмосферу дома.

Changes:

Моя → Мой (feminine → masculine).
сестра → брат (sister → brother).

The rest of the sentence (любит создавать уютную атмосферу дома) stays exactly the same.