Mér verður illt í bakinu ef ég sit of lengi.

Because Icelandic often expresses physical feelings with the person in the dative case, not as the grammatical subject the way English does.

So:

• mér = to me / for me (dative of ég)
• literally, the sentence begins something like To me becomes...

This is a very common Icelandic pattern with sensations and bodily states. Compare:

• Mér er kalt = I am cold
  literally: To me is cold
• Mér verður illt í bakinu = My back starts to hurt / I get pain in my back

So even though English says I, Icelandic often says mér in this kind of sentence.

verður is from verða, which often means become, get, or sometimes helps form a future-like meaning.

Here, verður illt means something like:

• becomes painful
• starts to hurt
• gets sore

So Mér verður illt í bakinu is not just my back is painful, but more like:

• my back starts hurting
• I get back pain
• my back gets sore

It suggests a change of state.

Because illt is the neuter singular form of illur (bad, painful, unwell depending on context).

In this expression, Icelandic uses illt almost like an impersonal predicate:

• Mér verður illt = I feel bad / pain starts
• Mér verður illt í bakinu = I get pain in my back

So the neuter form is used because the construction is not agreeing directly with mér. This is a set pattern you should get used to:

• mér er illt
• mér verður illt
• henni er illt í höfðinu

Even though the person is masculine/feminine/plural, the adjective often stays in this neuter singular form in these impersonal expressions.

Here it means painful / sore / hurting, not morally bad.

The adjective illur can have a few related meanings depending on context:

• bad
• unwell
• painful / sore

In Mér verður illt í bakinu, the meaning is clearly about physical pain:

• My back hurts
• I get pain in my back

So in body-part expressions, illt often corresponds to hurts or is sore in English.

Because Icelandic commonly says that pain is in a body part:

• í bakinu = in the back
• í höfðinu = in the head
• í fætinum = in the leg

This is the natural Icelandic way to locate the pain.

So:

• Mér verður illt í bakinu = literally It becomes painful to me in the back

English usually prefers my back hurts, but Icelandic often uses this í + body part pattern.

Bakinu is the dative singular definite form of bak (back).

Breaking it down:

• bak = back
• baki = dative singular
• bakið = the back (nominative/accusative definite)
• bakinu = in/to/from the back, the back in dative

Since í takes the dative when it means in a location, you get:

• í bakinu = in the back

The -nu ending shows both:

1. dative case
2. definite article (the)

Because with í, Icelandic distinguishes between:

• location → usually dative
• motion into something → usually accusative

Here the meaning is location:

• pain in the back

So it uses the dative:

• í bakinu

Compare the general idea:

• Ég er í húsinu = I am in the house → location, dative
• Ég fer í húsið = I go into the house → motion into, accusative

In your sentence, nothing is moving into the back; the pain is located there.

Because Icelandic, like English, often uses the present tense for:

• general truths
• habitual situations
• predictable results

So:

• Mér verður illt í bakinu ef ég sit of lengi

means something like:

• My back hurts if I sit too long
• I get back pain if I sit too long
• My back starts hurting when I sit too long

It is not necessarily talking about one specific future event. It describes a regular consequence.

Sit is the 1st person singular present tense of sitja (to sit):

• ég sit = I sit / I am sitting

After ef (if), Icelandic often uses the normal indicative present when talking about a real or likely condition:

• ef ég sit of lengi = if I sit too long

So this is just the ordinary present-tense form, not a special conditional form.

Of lengi means too long.

• of = too, overly
• lengi = for a long time / long

So:

• sit of lengi = sit too long

Be careful: this of is not the same as English of. It means too in the sense of excess:

• of heitt = too hot
• of dýrt = too expensive
• of lengi = too long

It is an adverb.

It describes how long the action of sitting lasts:

• ég sit lengi = I sit for a long time
• ég sit of lengi = I sit too long

Since it modifies the verb sit, not a noun, it is adverbial.

You can do a rough literal translation, but it will sound unnatural in English.

A very literal version would be something like:

• To me becomes painful in the back if I sit too long

That helps show the grammar, but natural English would be:

• My back hurts if I sit too long
• I get back pain if I sit too long
• My back gets sore if I sit too long

Literal translation is useful for understanding the structure, but not for producing natural English.

It can express similar ideas in other ways, but this dative-style pattern is very idiomatic and common in Icelandic.

Icelandic often focuses on the experience happening to the person:

• Mér er illt í bakinu
• Mér verður illt í bakinu

rather than making the back the subject.

So instead of structuring it like English My back hurts, Icelandic often prefers something more like:

• It hurts to me in the back
• Pain happens to me in the back

That is just one of the places where Icelandic and English organize experience differently.

This is an important difference:

• Mér er illt í bakinu = My back hurts / I have pain in my back
• Mér verður illt í bakinu = My back starts to hurt / My back gets sore

So:

• er describes the state as already true
• verður describes the state as arising or happening

In your sentence, verður fits well because sitting too long causes the pain to appear.

Because Icelandic allows either order, just like English:

• Mér verður illt í bakinu ef ég sit of lengi.
• Ef ég sit of lengi, verður mér illt í bakinu.

Both are fine.

If the ef clause comes first, notice the usual Icelandic word order change in the main clause:

• Ef ég sit of lengi, verður mér illt í bakinu.

The verb verður comes before mér because Icelandic is generally a verb-second language in main clauses.

Usually if, but in practice it can feel close to when in sentences about a regular consequence.

• ef ég sit of lengi = if I sit too long

Because the speaker probably means this happens reliably, English might sometimes also say:

• When I sit too long, my back hurts

But grammatically, ef is the normal word for if.

Most naturally, it describes a general repeated situation:

• whenever I sit too long, my back starts hurting

The present tense and the ef clause make it sound like a typical consequence, not necessarily one single event.

So a learner should hear it as something like:

• If I sit too long, my back hurts
• I get back pain if I sit too long

Yes, very often. This is a productive and useful pattern.

Examples:

• Mér er illt í höfðinu. = My head hurts.
• Henni verður illt í maganum. = Her stomach starts to hurt.
• Þeim er illt í fótunum. = Their legs hurt.

So the pattern is:

• [dative person] + er/verður illt + í + dative body part

That is a very common way to talk about pain in Icelandic.