У мене болить спина, бо я весь день сидів за комп’ютером.

Because Ukrainian often expresses pain as something happening to a person, not something the person actively does.

So У мене болить спина is literally closer to My back is hurting / At my place, the back hurts than to I hurt my back.

• я = I
• у мене = with me / in my case / I have

This pattern is very common with body parts and symptoms:

• У мене болить голова. = My head hurts.
• У мене болять очі. = My eyes hurt.

Because болить agrees with спина, which is the thing that hurts.

In this sentence, спина is the grammatical subject:

• спина болить = the back hurts

So the verb is 3rd person singular:

• болить = hurts / aches

If the painful thing were plural, you would use болять:

• У мене болять ноги. = My legs hurt.

Using я with боліти would not be the normal way to say my back hurts.

Because У мене already tells you whose back it is.

So Ukrainian does not need to say моя спина here.
У мене болить спина already naturally means my back hurts.

You can say моя спина, but it usually adds emphasis or contrast:

• У мене болить моя спина would sound unnecessary in most contexts.
• Не рука, а моя спина болить. = It’s not my arm, it’s my back that hurts.

Спина is in the nominative case because it is the subject of the verb болить.

The structure is:

• спина = subject
• болить = verb
• у мене = experiencer

So the grammar is basically:

• The back hurts, in my case.

That is why спина is not in the accusative.

Бо means because.

Yes, you can also use тому що:

• У мене болить спина, бо я весь день сидів за комп’ютером.
• У мене болить спина, тому що я весь день сидів за комп’ютером.

Both are correct.

A rough difference:

• бо = shorter, very common, often a bit more conversational
• тому що = fuller, neutral, also very common

Because бо introduces a subordinate clause.

The sentence has two parts:

• У мене болить спина
• бо я весь день сидів за комп’ютером

In standard Ukrainian spelling, you normally put a comma before conjunctions like бо, тому що, коли, якщо, etc. when they introduce a clause.

Весь день means all day / the whole day.

Here it is in the accusative case, used to show duration of time.

This is a common Ukrainian pattern:

• чекати цілу годину = to wait a whole hour
• працювати весь день = to work all day
• спати всю ніч = to sleep all night

Because день is masculine inanimate, its accusative form looks like the nominative here:

• nominative: день
• accusative: день

Сидів is the past tense, and the ending tells you the speaker is masculine singular.

So:

• сидів = I/he sat / was sitting if the speaker is male
• сиділа = if the speaker is female
• сиділи = if the subject is plural

In Ukrainian past tense, verbs agree with gender and number:

• Я сидів = a man is speaking
• Я сиділа = a woman is speaking

Because сидіти is an imperfective verb, and it fits the idea of an ongoing activity over a period of time.

The sentence describes a state/activity lasting through the day:

• я весь день сидів = I was sitting all day / I sat all day

This is very natural in Ukrainian.

You could also hear:

• я весь день просидів за комп’ютером

That adds a stronger sense of I spent the whole day sitting at the computer.

So:

• сидів = neutral, natural
• просидів = emphasizes the completed stretch of time spent sitting

Because сидіти за комп’ютером is the normal Ukrainian expression for to sit at the computer.

This is similar to:

• сидіти за столом = sit at the table
• працювати за столом = work at the desk

Some comparisons:

• за комп’ютером = at the computer, using it, working/sitting there
• перед комп’ютером = physically in front of the computer
• на комп’ютері = on the computer, often focusing on the device/software rather than the seated position

So after сидіти, за комп’ютером is the most idiomatic choice.

Комп’ютером is in the instrumental case.

The preposition за often takes:

• instrumental for location: at / behind
• accusative for movement: to a position behind / beyond

Here there is no movement; the person is already sitting there. So Ukrainian uses за + instrumental:

• за комп’ютером
• за столом
• за кермом = at the wheel / driving

The apostrophe in Ukrainian is part of the spelling. It is not a quotation mark here.

It shows that the consonant before it should not blend softly with the following vowel, and that ю keeps its y/u sound.

So комп’ютер is pronounced roughly like:

• kom-PYU-ter

The apostrophe is common in words like:

• ім’я = name
• п’ять = five
• об’єкт = object

No, Ukrainian word order is fairly flexible.

The version you have is neutral and natural:

• У мене болить спина, бо я весь день сидів за комп’ютером.

But other orders are possible for emphasis:

• У мене спина болить, бо я весь день сидів за комп’ютером.
• Бо я весь день сидів за комп’ютером, у мене болить спина.

The meaning stays basically the same, but the focus changes. For learners, the original order is the best default one to use.