Við byrjum ræktunina með því að sá snemma.

Byrjum is the 1st person plural present tense of að byrja (to begin / to start).

• við = we
• við byrjum = we begin / we start

So the verb agrees with the subject við.

A few related forms:

• ég byrja = I begin
• þú byrjar = you begin
• við byrjum = we begin
• þeir byrja = they begin

Ræktunina is the word ræktun (cultivation, growing, farming/cultivation process) with the definite article attached and in the accusative singular.

Breakdown:

• ræktun = cultivation
• ræktunin = the cultivation
• ræktunina = the cultivation (accusative form)

It is accusative because it is the direct object of byrjum in this sentence:

• Við byrjum ræktunina ... = We start the cultivation ...

Since ræktun is a feminine noun, its definite forms change by case.

Með því að is a very common Icelandic structure meaning by doing something.

So:

• með því að sá snemma = by sowing early

It introduces the method or means by which something is done.

You can think of it roughly like this:

• með = with
• því = that / it
• að + infinitive = to do something

But it is best learned as one expression:

• með því að + infinitive = by + -ing in English

Examples:

• Hann lærir íslensku með því að lesa á hverjum degi.
  He learns Icelandic by reading every day.

• Við sparum tíma með því að vinna saman.
  We save time by working together.

In this phrase, því is part of the fixed expression með því að. On its own, því is a form of the pronoun það, but in this construction you usually do not need to translate it word-for-word.

The important thing for learners is:

• með því að + infinitive = by doing ...

So although því has a grammatical role historically and structurally, in practice you should treat the whole phrase as a unit.

Because here sá is the infinitive of að sá, which means to sow.

This can confuse learners because sá can also be a form of að sjá (to see), namely the past tense:

• ég sá = I saw

But in your sentence, sá comes after með því að, which is followed by an infinitive, so it must mean:

• að sá = to sow

So:

• með því að sá snemma = by sowing early

not

• by saw early
• or by seeing early

Context and grammar make the meaning clear.

There actually is an að in the phrase með því að sá snemma. The að belongs to the construction and introduces the infinitive sá.

In Icelandic, infinitives are often used with að, but in some set expressions the structure may feel more tightly connected than in English. Here the full pattern is:

• með því að + infinitive

So:

• með því að sá = by sowing

Snemma is an adverb, meaning early.

It modifies the verb sá:

• að sá snemma = to sow early

Compare:

• snemma = early
• seint = late

Examples:

• Við komum snemma. = We arrive early.
• Hann vaknar snemma. = He wakes up early.

It is not an adjective here, because it is describing when the action happens, not a noun.

Yes, it is grammatical and understandable. It means that the cultivation/growing process starts by sowing early.

That said, Icelandic often also uses byrja á að + infinitive for start by doing something:

• Við byrjum á að sá snemma.

This is also very natural and may sound more everyday in some contexts.

The original sentence is slightly more explicit because it mentions ræktunina directly:

• Við byrjum ræktunina ... = We begin the cultivation ...

Yes, Icelandic word order is somewhat flexible, although the given version is neutral and clear.

For example, you could move snemma for emphasis in some contexts, but the original sentence is a straightforward default order:

• Við byrjum ræktunina með því að sá snemma.

That order presents the information in a very natural sequence:

1. subject: Við
2. verb: byrjum
3. object: ræktunina
4. method: með því að sá snemma

So for a learner, this is a good model sentence to follow.

Here are the main dictionary forms:

• við = we
• að byrja = to begin, to start
• ræktun = cultivation, growing
• með = with, by
• það = it, that
• að sá = to sow
• snemma = early

And here are the actual forms used in the sentence:

• við
• byrjum ← from að byrja
• ræktunina ← from ræktun
• með
• því ← from það
• að sá
• snemma