Ræktunin og moltugerðin bæta vaxtarskilyrðin.

Because Icelandic usually puts the definite article the onto the end of the noun instead of using a separate word.

So:

• ræktun → ræktunin = the cultivation
• moltugerð → moltugerðin = the composting / the making of compost
• vaxtarskilyrði → vaxtarskilyrðin = the growing conditions

The exact article ending changes with gender, number, and case. Even though you see -in more than once here, it is not always the same grammatical ending.

Because the subject is plural in meaning: ræktunin og moltugerðin = the cultivation and the composting.

In Icelandic, a subject joined by og usually takes a plural verb.

So:

• bætir = singular present
• bæta = plural present

Compare:

• Ræktunin bætir vaxtarskilyrðin. = The cultivation improves the growing conditions.
• Ræktunin og moltugerðin bæta vaxtarskilyrðin. = The cultivation and the composting improve the growing conditions.

Here it is a finite verb: present tense, plural.

This can be confusing because the infinitive is also bæta. Icelandic often has forms that look identical.

You can tell it is a finite verb here because it is the main verb of the sentence and agrees with the subject:

• að bæta = to improve / infinitive
• þau bæta = they improve / present plural

In this sentence, bæta means improve and is the actual verb of the clause.

It is the direct object, so it is in the accusative.

The verb bæta when it means improve takes an accusative object.

So the structure is:

• ræktunin og moltugerðin = subject
• bæta = verb
• vaxtarskilyrðin = object

One extra detail: vaxtarskilyrðin is neuter plural definite, and for many neuter nouns the nominative and accusative plural look the same. So the form itself does not visibly change here, but its role in the sentence is still accusative.

The nouns are:

• ræktun — feminine
• moltugerð — feminine
• vaxtarskilyrði — neuter

That helps explain the forms:

• ræktunin = feminine nominative singular definite
• moltugerðin = feminine nominative singular definite
• vaxtarskilyrðin = neuter plural definite

A useful point here is that the ending -in can represent different things:

• feminine singular definite
• neuter plural definite

So you have to read the whole word, not just the last letters.

Because both nouns are definite.

ræktunin og moltugerðin means the cultivation and the composting, where both parts are specific/definite. In Icelandic, you normally mark each noun separately.

So:

• ræktunin og moltugerðin = both definite
• ræktun og moltugerð = both indefinite

You usually cannot treat the article as if it only applies once to the whole pair.

Because Icelandic compounds often use the first noun in a linking form, very often the genitive form.

The noun is molta = compost. Its genitive singular is moltu. That is the form used in the compound:

• molta
• moltu-
  • gerð
• moltugerð = compost-making / compost production
• moltugerðin = the compost-making

This is very common in Icelandic compounds: the first part is often not the bare dictionary form.

It is a compound noun:

• vaxtar- = growth-
• skilyrði = condition / conditions

So vaxtarskilyrði literally means growth conditions.

The first part vaxtar- is a combining form related to growth. Icelandic very often makes compounds like this, where the first element modifies the second.

Then the definite ending is added:

• vaxtarskilyrði = growing conditions
• vaxtarskilyrðin = the growing conditions

Because some Icelandic neuter nouns have the same form in indefinite singular and indefinite plural.

So:

• skilyrði can mean a condition
• skilyrði can also mean conditions

Context tells you which one is meant.

The definite forms help distinguish them:

• skilyrðið = the condition (singular)
• skilyrðin = the conditions (plural)

In your sentence, vaxtarskilyrðin is plural: the growing conditions.

Very often, yes. This sentence is a straightforward subject + verb + object pattern:

• Ræktunin og moltugerðin = subject
• bæta = verb
• vaxtarskilyrðin = object

So for an English speaker, the word order here feels familiar.

That said, Icelandic is more flexible than English and follows a verb-second pattern in main clauses, so other word orders are possible in different contexts. But this sentence is the basic, neutral order.

Two letters here are especially worth noticing:

• æ is usually pronounced roughly like the vowel in eye
• ð is usually like the th in this

So:

• bæta sounds roughly like BYE-ta
• gerð has a soft th-like sound at the end because of ð

These are only rough English approximations, but they are good starting points for a learner.