Вчера я немного обгорела на плечах, поэтому сегодня сижу в тени и пью воду.

Because the verb is in the past tense, and in Russian past tense verbs agree with the speaker’s gender.

• обгорел = a man is speaking
• обгорела = a woman is speaking
• обгорело = neuter
• обгорели = plural

So я немного обгорела tells you that the speaker is female.

Обгореть means to get sunburned or more literally to get burned from the sun / heat on the surface.

In this sentence, it clearly means to get sunburned.

A useful contrast:

• сгореть = to burn up, be destroyed by fire, or sometimes to get very badly sunburned
• обгореть = to get burned on the outside, often used for sunburn

So немного обгорела means got a little sunburned.

Немного means a little, slightly, or somewhat.

In я немного обгорела, it softens the statement:

• я обгорела = I got sunburned
• я немного обгорела = I got a little sunburned

Its position before the verb is very natural. In Russian, adverbs like немного often go before the verb they modify.

Because на плечах is a fixed and very natural way to say on the shoulders.

Here:

• на = on
• плечах = prepositional plural of плечо (shoulder)

Why plural? Because people usually mean both shoulders together.

So:

• обгорела на плечах = got sunburned on my shoulders

Compare:

• на плечах = on the shoulders, location
• на плечи = onto the shoulders, direction/motion

Since there is no movement here, Russian uses the prepositional case after на.

Russian often leaves out possessive words like мой / моя / мои when the owner is obvious from context.

Here, since the speaker says я and is talking about their own body, Russian does not need to say на моих плечах.

Both are possible, but they feel different:

• обгорела на плечах = natural, normal
• обгорела на моих плечах = more explicit than necessary

This is very common with body parts in Russian.

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

In this sentence:

• Вчера я немного обгорела на плечах, поэтому сегодня сижу в тени и пью воду.
• Yesterday I got a little sunburned on my shoulders, so today I’m sitting in the shade and drinking water.

It often connects cause and result:

• cause: Вчера я немного обгорела на плечах
• result: поэтому сегодня сижу в тени и пью воду

It commonly appears near the start of the second clause, just like here.

Because the sentence contrasts yesterday with today:

• Вчера = yesterday → past tense: обгорела
• сегодня = today → present tense: сижу, пью

So the meaning is:

• yesterday something happened
• today the speaker is reacting to it

This mix of tenses is perfectly normal because the time references are different.

Сидеть is the dictionary form (infinitive), meaning to sit.

Сижу is the 1st person singular present tense:

• я сижу = I am sitting / I sit

Some present tense forms of сидеть:

• я сижу
• ты сидишь
• он / она сидит
• мы сидим
• вы сидите
• они сидят

So in the sentence, сижу matches я.

Because в тени means in the shade, which is a location, not movement toward something.

Russian uses:

• в + prepositional for location
• в + accusative for motion into something

So:

• сижу в тени = I am sitting in the shade
• иду в тень = I am going into the shade

Here the speaker is already located there, so в тени is correct.

Russian can say either, but пью воду is more explicit.

• пью = I’m drinking
• пью воду = I’m drinking water

Adding воду makes the sentence clearer and more natural in this context, especially because drinking water is part of recovering from sun exposure.

Also, воду is in the accusative case because it is the direct object of пью.

Because вода is the nominative form, but after the verb пить you need the accusative for the direct object.

• nominative: вода
• accusative: воду

So:

• Я пью воду = I drink / am drinking water

This is a very common pattern:

• читать книгу = to read a book
• есть суп = to eat soup
• пить воду = to drink water

No, Russian word order is fairly flexible, but the version given is very natural and neutral.

Original:

• Вчера я немного обгорела на плечах, поэтому сегодня сижу в тени и пью воду.

This order works well because:

• Вчера sets the past-time background
• я introduces the subject
• поэтому smoothly introduces the result
• сегодня highlights the contrast with вчера

You could move some parts around, but the emphasis would change. For example:

• Сегодня сижу в тени и пью воду, потому что вчера немного обгорела на плечах.

That is also correct, but the structure is different: it starts with today’s behavior and then gives the reason.

Yes, but the sentence structure would change.

• поэтому = therefore / so
• потому что = because

With поэтому, you say:

• cause first, result second
• Вчера я немного обгорела на плечах, поэтому сегодня сижу в тени и пью воду.

With потому что, you would usually say:

• result first, then cause
• Сегодня сижу в тени и пью воду, потому что вчера немного обгорела на плечах.

Both are natural, but they organize the information differently.

Yes, it is natural. Russian often describes sunburn by mentioning the body part with на:

• обгореть на носу = get sunburned on the nose
• обгореть на спине = get sunburned on the back
• обгореть на плечах = get sunburned on the shoulders

You may also hear related expressions like:

• сгореть на солнце = to get badly sunburned in the sun
• у меня обгорели плечи = my shoulders got sunburned

So this sentence uses a normal, idiomatic pattern.