Эта майка из хлопка, поэтому в ней не жарко даже днём.

Because майка is a feminine noun in Russian, and the demonstrative this must agree with it in gender.

• masculine: этот
• feminine: эта
• neuter: это

So:

• эта майка = this shirt / this tank top

Майка usually means a light upper-body garment such as a tank top, undershirt, or sometimes a casual T-shirt-like top, depending on context.

It is not the most general word for shirt. For example:

• рубашка = shirt, especially a button-up shirt
• футболка = T-shirt
• майка = tank top / sleeveless top / undershirt

So here майка suggests a light piece of clothing, which fits well with the idea that it is not hot to wear.

Because the preposition из normally takes the genitive case when it means made of / from.

• хлопок = cotton
• genitive singular: хлопка

So:

• из хлопка = made of cotton

This is a very common pattern in Russian:

• из дерева = made of wood
• из металла = made of metal
• из кожи = made of leather

Literally it is closer to from cotton, but in normal English we usually translate it as made of cotton.

In Russian, из + genitive is the standard way to say what something is made from.

So Эта майка из хлопка means:

• This top is made of cotton

Because поэтому here means therefore / so / that’s why and connects two clauses:

• Эта майка из хлопка
• поэтому в ней не жарко даже днём

In Russian, this kind of connector is commonly separated by a comma.

So the structure is:

• This top is made of cotton, therefore it isn’t hot in it even during the day.

Поэтому means therefore, that’s why, or so.

It shows a result:

• cause: Эта майка из хлопка = This top is made of cotton
• result: поэтому в ней не жарко = so it’s not hot in it

It is a very useful linking word in Russian for cause-and-result statements.

Russian often uses в + prepositional to talk about how someone feels in a piece of clothing.

So:

• в ней = in it
• literally: in this top

This sounds natural in Russian, even though in English we often say:

• it’s not hot wearing it
• it’s not too hot in this top
• you don’t feel hot in it

Here ней refers back to майка, which is feminine.

Compare:

• В этой куртке тепло. = It’s warm in this jacket.
• В этих ботинках удобно. = These shoes are comfortable.

The form ней can mean her or it, depending on context.

In this sentence, it refers to майка, because майка is feminine. Russian uses the same pronoun forms for feminine nouns and for she/her.

So:

• майка = feminine noun
• в ней = in it (referring to the shirt)

Russian does this all the time:

• книга is feminine → в ней = in it
• сумка is feminine → в ней = in it

The context tells you it is not about a person.

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

Russian often uses special predicative words ending in -о in impersonal sentences:

• жарко = hot
• холодно = cold
• тепло = warm
• темно = dark

So:

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

This is an impersonal construction. There is no explicit subject like it in English.

Даже днём means even during the day or even in the daytime.

The word днём is the instrumental singular form of день and is used in a time expression.

Some common time expressions in Russian use the instrumental case:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So here:

• даже днём = even during the day, when it is typically warmer

Yes, it matters for pronunciation and sometimes for clarity.

• днём is pronounced with yo
• stress falls on ём

In normal printed Russian, ё is sometimes replaced by е, so you may also see днем, but the meaning is still днём.

For learners, it is helpful to remember the real pronunciation:

• днём = dyom

Usually в ней is the more natural choice here when talking about being dressed in a garment and how it feels on the body.

Russian often prefers:

• в куртке
• в платье
• в майке

when talking about comfort, temperature, or appearance while wearing clothes.

На ней usually means on her or on it, and with clothing it often refers more to the item being physically on someone, not so much the experience of wearing it.

So for this sentence, в ней не жарко is the normal and idiomatic phrasing.

The sentence is natural as written, but Russian word order is fairly flexible.

Original:

• Эта майка из хлопка, поэтому в ней не жарко даже днём.

You could also hear variations like:

• Эта майка из хлопка, поэтому даже днём в ней не жарко.

This version puts extra emphasis on even during the day.

In Russian, word order often changes to shift emphasis rather than basic meaning.

A few very common Russian patterns appear here:

1. Demonstrative agrees with noun

   • эта майка

2. из + genitive for material

   • из хлопка

3. поэтому for result

   • therefore / so

4. в + prepositional for how something feels when wearing clothes

   • в ней

5. impersonal predicate for conditions/feelings

   • не жарко

6. instrumental for time expressions

   • днём

So this sentence is a very useful example of several everyday Russian structures working together.