Летом моя дочка любит спать в мягкой футболке, потому что в ней не жарко.

In Russian, some time expressions are commonly used without a preposition, and летом is one of them. It means in summer / during the summer.

This is a very common pattern with seasons:

• зимой — in winter
• весной — in spring
• летом — in summer
• осенью — in autumn

These forms are historically instrumental and are best learned as fixed adverb-like time expressions.

Both mean my daughter, but they are slightly different in tone.

• дочь is the basic, neutral word: daughter
• дочка is a more affectionate, everyday form: little daughter / daughter in a warm tone

So моя дочка sounds more natural and tender in casual speech, especially when talking about one’s child.

After любить, Russian often uses an infinitive to say what someone likes doing.

So:

• любить читать — to like reading
• любить гулять — to like walking
• любить спать — to like sleeping

Literally, любит спать is likes to sleep.

Russian does not need a special -ing form here the way English often does.

Because the preposition в here means in / wearing, and that idea often takes the prepositional case when talking about being in something.

Футболка is feminine, so in the prepositional case:

• мягкая футболка → nominative
• в мягкой футболке → prepositional

The adjective must match the noun:

• мягкая → мягкой
• футболка → футболке

So the whole phrase changes together.

Literally it is in a soft T-shirt, but in natural English the meaning is wearing a soft T-shirt.

Russian often uses в + clothing item to describe what someone is dressed in:

• Он в куртке — He is wearing a jacket
• Она в платье — She is wearing a dress

So спать в мягкой футболке means to sleep in a soft T-shirt.

Ней refers back to футболке.

Since футболка is feminine, the pronoun is:

• nominative: она
• prepositional after в: ней

So:

• в ней = in it / in her, but here it clearly means in it, referring to the T-shirt

The phrase потому что в ней не жарко means because it’s not hot in it / because she doesn’t feel hot in it.

Because жарко here is not describing the T-shirt itself. It describes the feeling or condition: it is hot / one feels hot.

Russian often uses short impersonal predicate words like:

• жарко — hot
• холодно — cold
• трудно — difficult
• легко — easy

So:

• в ней не жарко = it’s not hot in it / you don’t feel hot in it

If you said она не жаркая, that would mean it (the T-shirt) is not hot/warm, describing the shirt as an object, which is not the intended idea here.

Потому что means because.

It introduces the reason:

• Летом моя дочка любит спать в мягкой футболке — In summer my daughter likes to sleep in a soft T-shirt
• потому что в ней не жарко — because it’s not hot in it

This is one of the most common ways to give a reason in Russian.

Yes. Russian word order is flexible, though the original version is very natural.

Original:

• Летом моя дочка любит спать в мягкой футболке, потому что в ней не жарко.

Possible variations:

• Моя дочка летом любит спать в мягкой футболке, потому что в ней не жарко.
• В мягкой футболке моя дочка летом любит спать, потому что в ней не жарко.

The basic meaning stays the same, but the focus shifts slightly. The original sounds smooth and neutral, with летом setting the scene right away.

Russian has no articles like a or the.

So в мягкой футболке could mean:

• in a soft T-shirt
• in the soft T-shirt

The exact interpretation comes from context. In this sentence, English would usually say in a soft T-shirt, because it is talking about the kind of clothing she likes to sleep in, not a specific previously mentioned T-shirt.

Yes, very common.

Russian often uses спать в + clothing item for what someone sleeps wearing:

• спать в пижаме — sleep in pajamas
• спать в носках — sleep in socks
• спать в футболке — sleep in a T-shirt

So this part of the sentence is very natural and idiomatic.

It is singular because the sentence talks about the type of clothing she likes to sleep in: a soft T-shirt.

Plural would change the meaning:

• в мягкой футболке — in a soft T-shirt
• в мягких футболках — in soft T-shirts

The plural version would sound less natural here unless you were talking generally about several different T-shirts she owns.

Here ней is the prepositional-case form of она, used after the preposition в.

The relevant forms are:

• она — nominative
• её / её — accusative/genitive
• ей — dative
• ею / ей — instrumental
• ней — prepositional (and also used after some prepositions in other contexts)

So в ней is a standard preposition + pronoun combination meaning in it when referring to a feminine noun like футболка.