Після обіду в мене болить живіт, якщо я їм надто швидко.

Because після requires the genitive case.

The base form is обід.
Its genitive singular form is обіду.

So:

• обід = lunch
• після обіду = after lunch

This is a very common pattern in Ukrainian:

• після уроку = after the lesson
• після роботи = after work
• після дощу = after the rain

In standard Ukrainian, обід usually means lunch, especially the main meal in the middle of the day.

So:

• сніданок = breakfast
• обід = lunch
• вечеря = dinner / supper

If you want to say after dinner, you would normally say після вечері.

Yes. в мене and у мене are both correct here.

Ukrainian often alternates в and у for euphony—to make the sentence sound smoother. The meaning does not change.

So both are natural:

• В мене болить живіт
• У мене болить живіт

In this sentence, в мене / у мене means something like for me or I have in English-style translation, but literally it is closer to at me.

Because this is the normal, idiomatic way to talk about pain in Ukrainian.

Ukrainian usually says:

• У мене болить голова = My head hurts
• У мене болить живіт = My stomach hurts
• У мене болить спина = My back hurts

Literally, the structure is closer to At me, the stomach hurts.

A phrase like я маю біль is understandable, but it sounds much less natural in everyday speech.

Because живіт is singular.

The verb agrees with the thing that hurts:

• болить = singular
• болять = plural

So:

• болить живіт = the stomach hurts
• болить голова = the head hurts
• болять очі = the eyes hurt
• болять ноги = the legs hurt

Here живіт is in the nominative case, because it is the grammatical subject of болить.

The structure is not I hurt the stomach.
It is the stomach hurts.

So:

• живіт болить = the stomach hurts

That is why the basic form живіт appears.

Also, Ukrainian often does not need a separate word for my here, because в мене / у мене already shows whose stomach it is.

Compare:

• У мене болить живіт = My stomach hurts
• Мій живіт болить = My stomach hurts, but this sounds more emphatic or contrastive

їм is the 1st person singular present tense of їсти, which means to eat.

So:

• я їм = I eat
• ти їси = you eat
• він / вона їсть = he / she eats
• ми їмо = we eat
• ви їсте = you eat
• вони їдять = they eat

This verb is somewhat irregular, so it is worth memorizing.

In the sentence:

• якщо я їм надто швидко = if I eat too quickly

Not exactly.

• надто швидко = too quickly / excessively quickly
• дуже швидко = very quickly

So надто suggests that the speed is a problem.
That fits this sentence, because eating too fast causes the stomach pain.

You may also hear занадто швидко, which is very close in meaning to надто швидко:

• надто швидко = too quickly
• занадто швидко = too quickly
• дуже швидко = very quickly

Because якщо introduces a subordinate clause.

In Ukrainian, subordinate clauses are normally separated by commas.

So:

• Після обіду в мене болить живіт, якщо я їм надто швидко.

You can also reverse the order:

• Якщо я їм надто швидко, після обіду в мене болить живіт.

The comma is still needed.

Yes. Ukrainian word order is fairly flexible.

The given sentence is natural:

• Після обіду в мене болить живіт, якщо я їм надто швидко.

But you could also say:

• Якщо я їм надто швидко, після обіду в мене болить живіт.

Both mean the same thing. The difference is mostly about focus and flow:

• starting with Після обіду highlights the time first
• starting with Якщо я їм надто швидко highlights the condition first

Yes, often you can.

Because їм already shows 1st person singular, Ukrainian does not always need the pronoun я.

So both are possible:

• якщо я їм надто швидко
• якщо їм надто швидко

Including я is still perfectly natural. It can make the sentence a little clearer or slightly more explicit.

They are related, but not exactly the same structure.

• У мене болить живіт = My stomach hurts
• У мене біль у животі = I have pain in my stomach / I have stomach pain

The first uses a verb: болить.
The second uses a noun: біль.

In everyday speech, У мене болить живіт is the most straightforward and common way to say it.