Mam już kupione nasiona i przekopaną ziemię, więc jutro zaczniemy pracę wcześniej.

These two versions are close in meaning, but they focus on different things.

• kupiłem/kupiłam... przekopałem/przekopałam... = a simple past statement about what you did.
• mam... kupione... przekopaną... = emphasis on the resulting state now.

So mam już kupione nasiona i przekopaną ziemię means something like:

• I already have the seeds bought
• I already have the soil dug over
• in natural English: I already have the seeds bought and the soil dug over / I’ve already got the seeds bought and the soil dug over

This construction is very common in Polish when the important idea is the preparation is already done.

They are past passive participles, used a bit like adjectives.

They come from verbs:

• kupić → kupiony / kupiona / kupione
• przekopać → przekopany / przekopana / przekopane

In this sentence, they describe the nouns:

• kupione nasiona = bought seeds
• przekopaną ziemię = dug-over soil

Because they behave like adjectives, they must agree with the noun in:

• gender
• number
• case

Because the participle has to match the noun it describes.

1. kupione nasiona

• nasiona = plural, non-masculine-personal
• so the participle takes the form kupione

2. przekopaną ziemię

• ziemię = feminine singular accusative
• so the participle takes the form przekopaną

So the endings are different because the nouns are grammatically different.

Because both are direct objects, so they are in the accusative case.

After mam

• mam ziemię = I have the soil
• therefore: mam przekopaną ziemię

After zaczniemy

• zaczniemy pracę = we will start the work
• so praca changes to pracę

This is a very normal accusative pattern in Polish.

Because for this noun, the nominative plural and accusative plural are the same.

• dictionary form singular: nasiono
• plural nominative: nasiona
• plural accusative: nasiona

So even though nasiona is a direct object here, it still looks unchanged.

This is very common in Polish with many neuter and inanimate nouns.

Not necessarily.

This construction mainly tells you that the task is done and the result is available now. It does not strongly focus on the agent.

So mam kupione nasiona can mean:

• I bought the seeds myself
• I got the seeds bought
• the seeds are already bought / taken care of

If you want to clearly say I bought them, Polish more directly uses:

• Kupiłem nasiona (male speaker)
• Kupiłam nasiona (female speaker)

So mam kupione is more about the state/result, not about who did it.

Już means already.

In Mam już kupione nasiona..., it naturally modifies the whole idea: the preparation has already been done.

This placement is very normal and neutral.

Other placements are possible, for example:

• Już mam kupione nasiona...
  Slightly more emphasis on already
• Mam kupione już nasiona...
  Possible, but less neutral in many contexts

So the sentence’s word order is a very natural default choice.

Więc means so, therefore, or thus.

It connects the two parts logically:

• I already have the seeds bought and the soil dug over
• so tomorrow we’ll start work earlier

The comma is standard before więc when it links clauses like this.

So:

• ..., więc jutro zaczniemy...

is exactly what you would expect in normal written Polish.

It is future.

The verb is zacząć = to begin / to start, which is a perfective verb.

In Polish, perfective verbs do not form the future with będę + infinitive. Instead, they use forms that look like present tense forms, but actually refer to the future:

• zacznę = I will start
• zaczniemy = we will start

So jutro zaczniemy pracę means tomorrow we will start work.

This is completely normal Polish grammar.

Because Polish verb endings usually already show the subject.

• zaczniemy = we will start

The ending -my tells you it is 1st person plural: we.

So adding my is usually unnecessary unless you want emphasis or contrast:

• My zaczniemy wcześniej, a oni później.
  We will start earlier, and they later.

In your sentence, no pronoun is needed.

Because wcześniej is an adverb, and it modifies the verb zaczniemy.

It tells you how/when the work will start:

• zaczniemy wcześniej = we’ll start earlier

An adjective like wcześniejszą would have to describe a noun. But here the idea is not an earlier work; it is starting earlier.

Also, wcześniej is the comparative-type adverb form related to wcześnie (early), so it naturally means earlier.

Here praca means work in the sense of the task/activity to be done, not necessarily a job or employment.

So:

• zaczniemy pracę = we’ll start the work / we’ll get started on the work

In English, we often just say start work without an article. In Polish, pracę is a normal direct object here.

Polish word order is fairly flexible, but the original version sounds very natural:

• Mam już kupione nasiona i przekopaną ziemię, więc jutro zaczniemy pracę wcześniej.

You could rearrange parts for emphasis, for example:

• Już mam kupione nasiona i przekopaną ziemię, więc jutro zaczniemy pracę wcześniej.
• Jutro zaczniemy pracę wcześniej, bo mam już kupione nasiona i przekopaną ziemię.

But the original order is smooth and neutral:

1. first the preparation is mentioned
2. then the consequence follows with więc

So yes, the order can change, but this version is a very standard and natural one.