Najpierw przekopuję ziemię łopatą, a potem moja siostra bierze grabie.

This is a very common way to show sequence in Polish:

• najpierw = first / at first
• potem = then / afterwards
• a here links the two parts and often sounds like and then / while / and

So Najpierw ..., a potem ... means First ..., and then ....

Polish often uses this kind of time-order wording very naturally, especially when describing steps in a process.

Przekopuję comes from przekopywać / przekopać, which means something like:

• to dig over
• to turn over the soil
• to dig through

That is more specific than kopać (to dig).

So:

• kopię ziemię = I am digging the earth/soil
• przekopuję ziemię = I am digging over the soil, turning it over

In a gardening context, przekopywać ziemię is a very natural phrase.

Also, przekopuję is 1st person singular present tense: I dig over / I am digging over.

Because ziemię is in the accusative case.

The verb przekopuję takes a direct object, and the direct object usually goes into the accusative:

• ziemia = nominative form
• ziemię = accusative form

So:

• ziemia jest mokra = the soil is wet
• przekopuję ziemię = I am digging over the soil

This is a very common pattern in Polish: the thing directly affected by the action often appears in the accusative.

Because łopatą is the instrumental case, used here to show the tool used to do something.

• łopata = shovel/spade
• łopatą = with a shovel / using a shovel

So:

• przekopuję ziemię łopatą = I dig over the soil with a shovel

In Polish, the instrumental is often used without a preposition when you mean by means of / using something.

Other examples:

• piszę długopisem = I write with a pen
• jadę autobusem = I go by bus
• kroję nożem = I cut with a knife

Grabie is a noun that is normally used only in the plural. It means rake.

So even though English says a rake, Polish says grabie, which looks grammatically plural.

That means:

• dictionary form: grabie
• accusative form: also grabie

So:

• moja siostra bierze grabie = my sister takes the rake

This is one of those Polish nouns that are plural in form but singular in meaning in English.

Similar idea in English:

• scissors
• trousers

Polish has quite a few nouns like this.

Because grabie is the object, not the subject.

The subject is:

• moja siostra = my sister

Since moja siostra is singular, the verb must also be singular:

• moja siostra bierze = my sister takes

The fact that grabie has a plural form does not change the verb, because it is the thing being taken, not the person doing the action.

Bierze is the 3rd person singular present tense of brać (to take).

Here it means something like:

• takes
• picks up
• gets hold of

So moja siostra bierze grabie means that after I dig the soil, my sister takes/picks up the rake.

In context, it suggests the next step in the gardening process.

Both are possible, but moja siostra is more explicit: my sister.

• siostra = sister
• moja siostra = my sister

Polish often includes possessives when the speaker wants to be clear or natural in context. It is not strange at all here.

Also note agreement:

• moja matches siostra
• both are feminine singular nominative

Yes. Polish word order is more flexible than English word order, although the original sentence is very natural.

The original:

• Najpierw przekopuję ziemię łopatą, a potem moja siostra bierze grabie.

Possible variations include:

• Najpierw łopatą przekopuję ziemię, a potem moja siostra bierze grabie.
• Potem moja siostra bierze grabie, ale najpierw przekopuję ziemię łopatą.

But the original version is probably the most neutral and easiest for a learner:

• time word first
• action next
• second step introduced after that

So yes, the order can change, but Polish uses word order to shift emphasis, not just to mark grammar.

Yes, both verbs are in the present tense:

• przekopuję = I dig over / I am digging over
• bierze = she takes / she is taking

In Polish, the present tense can be used to describe:

• a habitual action
• a routine
• a set of steps
• a vivid description of what is happening

So this sentence can sound like:

• a description of what usually happens
• instructions told in a personal way
• a live description of the process

English can do something similar:

• First I dig over the soil, then my sister takes the rake.

This sentence is a nice example of three different cases:

1. Nominative

   • moja siostra
     This is the subject of bierze.

2. Accusative

   • ziemię
     Direct object of przekopuję
   • grabie
     Direct object of bierze

3. Instrumental

   • łopatą
     Shows the tool used: with a shovel

So the sentence is useful because it shows how Polish changes noun forms depending on their role in the sentence.