Þetta er góður háttur til að læra íslensku.

It breaks down like this:

• Þetta = this
• er = is
• góður háttur = a good way / a good method
• til að læra íslensku = to learn Icelandic / in order to learn Icelandic

So the sentence structure is:

• subject: Þetta
• verb: er
• subject complement: góður háttur
• purpose phrase: til að læra íslensku

A very literal breakdown would be:

This is [a] good way to learn Icelandic.

Because Icelandic does not have an indefinite article like English a/an.

So:

• góður háttur can mean a good way
• góði hátturinn would mean the good way

In other words, Icelandic often expresses indefiniteness with no article at all.

Þetta means this.

It is the neuter singular form of the demonstrative pronoun þessi. In sentences like this, þetta often works as a general this meaning:

• this thing
• this idea
• this method
• this situation

So even though háttur is masculine, þetta can still be neuter because it is not directly agreeing with háttur as an adjective would. It is standing on its own as the subject: This is ...

Because góður has to agree with háttur.

The noun háttur is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives góður.

Compare:

• góður háttur = masculine
• góð leið = feminine
• gott ráð = neuter

This is normal Icelandic adjective agreement.

Because it comes after the verb vera (to be) in a sentence like X is Y.

In Icelandic, after vera, the complement is usually in the nominative:

• Þetta er góður háttur.
• Hann er kennari.
• Hún er góð vinkona.

So both Þetta and góður háttur are in the nominative here.

Háttur means something like:

• way
• manner
• method

In this sentence, góður háttur means a good way or a good method.

A learner might also see other words in similar contexts, such as:

• leið = way, route, means
• aðferð = method, procedure

So góður háttur is correct, but Icelandic may also use other nouns depending on style and context.

Til að means to or in order to when expressing purpose.

So:

• til að læra íslensku = to learn Icelandic
• more literally: in order to learn Icelandic

This is a very common Icelandic pattern.

Examples:

• Hún fór út til að kaupa mjólk. = She went out to buy milk.
• Ég æfi mig til að verða betri. = I practice to become better.

Normally, til by itself takes the genitive:

• til Íslands = to Iceland
• til mín = to me

But in til að + infinitive, you should think of it as a fixed expression meaning in order to.

So in this sentence, að læra íslensku is not a genitive phrase after ordinary til. Instead, til að functions together as a purpose marker.

That is why it is best to learn til að + infinitive as one pattern.

Because after til að, Icelandic uses the infinitive.

So:

• að læra = to learn

Here, the infinitive phrase is:

• til að læra íslensku = to learn Icelandic

This is similar to English to learn, though Icelandic uses the full expression til að to show purpose.

Because læra takes a direct object, and that object is in the accusative.

The language name is:

• íslenska in the nominative
• íslensku in the accusative

So:

• Íslenska er erfið. = Icelandic is difficult.
  Here íslenska is nominative because it is the subject.
• Ég læri íslensku. = I am learning Icelandic.
  Here íslensku is accusative because it is the object.

That is exactly what is happening in your sentence:

• til að læra íslensku

For a learner, the simplest answer is: treat íslenska / íslensku here as the name of the language.

So you can learn it like this:

• íslenska = Icelandic (subject form)
• íslensku = Icelandic (object form)

Historically, language names are related to adjectives, and you may see that reflected in their forms. But in practice, learners usually do best by treating íslenska as the word for the Icelandic language, with its own case forms.

Because in Icelandic, language names are usually written with a lower-case letter.

So:

• íslenska = Icelandic
• enska = English
• danska = Danish

This is different from English, where language names are capitalized.

Þ / þ is pronounced like the th in thin, not like the th in this.

So Þetta begins with the voiceless th sound:

• þ = English th in thin

That letter is one of the first things English speakers notice in Icelandic. It is completely normal and very common.