Því meira sem ég les, því betur skil ég textann.

It is a very common Icelandic pattern meaning the more ..., the ... or more generally the X-er ..., the Y-er.

So:

• Því meira sem ég les = The more I read
• því betur skil ég textann = the better I understand the text

You can think of því ... því ... as a fixed correlative structure. English uses the ... the ..., while Icelandic uses því ... því ....

Here sem introduces the clause after the comparative expression.

So in:

• Því meira sem ég les

the part meira sem ég les is literally something like more that I read, but in natural English it becomes the more I read.

In this structure, sem is very normal after the comparative word:

• Því meira sem...
• Því minna sem...
• Því betur sem... sometimes possible depending on context

So you should learn því + comparative + sem + clause as a useful pattern.

Meira is the form normally used here because it works adverbially or as an abstract more without a noun after it.

Compare:

• meiri bók would be wrong, because adjectives must agree with nouns properly
• meiri áhugi = more interest (adjective agreeing with a noun)
• meira = more, when no noun is stated or when it functions more like an adverb/amount expression

In Því meira sem ég les, there is no noun after meira. It means something like the more (I read), not the bigger noun or the greater noun.

So meira is exactly the form you expect here.

Because betur is an adverb, while betri is an adjective.

Here, betur modifies the verb skil (understand), so the meaning is understand better.

• betri = better when describing a noun
  
  • betri bók = a better book
• betur = better when modifying a verb
  
  • Ég skil betur = I understand better

So in this sentence:

• því betur skil ég textann = the better I understand the text

Using betri here would be incorrect.

This is because Icelandic is a verb-second language.

In a main clause, the finite verb normally comes in the second position. If something other than the subject comes first, the verb still stays second, and the subject moves after it.

Here the sentence begins with:

• því betur

So that takes the first slot. The finite verb skil must then come second:

• því betur skil ég textann

Not:

• því betur ég skil textann ❌

This is very normal Icelandic word order.

A simpler comparison:

• Ég skil textann vel.
• Núna skil ég textann vel.

In the second sentence, Núna comes first, so the verb skil comes before ég. The same thing happens here.

Textann is accusative singular definite of texti (text).

Breakdown:

• texti = a text (indefinite nominative singular)
• textann = the text (definite accusative singular)

It has this form because skilja (to understand) takes a direct object, and that object is in the accusative:

• Ég skil textann. = I understand the text.

The ending -ann includes the definite article the attached to the noun, which is very common in Icelandic.

Historically, yes, it comes from the same form, but in this sentence you should treat it as part of a fixed expression rather than translating it separately.

In modern usage, því ... því ... functions as a set pattern meaning:

• the more ..., the more ...
• the more ..., the better ...
• the less ..., the less ...

So although því is also a normal Icelandic word in other contexts, here it is best learned as part of the construction, not as an independent word you translate one by one.

They are both 1st person singular present tense forms:

• ég les = I read
• ég skil = I understand

So the whole sentence is in the present tense and expresses a general truth or repeated experience:

• The more I read, the better I understand the text.

That is very natural in Icelandic, just as it is in English.

Not very naturally. A very literal version would be something like:

• By-that more that I read, by-that better understand I the text

That sounds strange in English, but it helps show the structure.

The natural English translation is:

• The more I read, the better I understand the text.

So this is a good example of a sentence where understanding the pattern is more useful than trying to translate each word separately.

Yes. This is a very productive pattern in Icelandic.

For example:

• Því meira sem ég æfi, því betri verð ég.
  The more I practice, the better I become.

• Því minna sem hann sefur, því þreyttari er hann.
  The less he sleeps, the more tired he is.

• Því fyrr sem þú kemur, því betra.
  The earlier you come, the better.

So the sentence you were given is not just one isolated expression; it shows a very useful grammar pattern you can reuse often.