Ziemniaki gotują się dłużej niż makaron, dlatego zaczynam je gotować wcześniej.

In this sentence, gotować się means to be cooking / to cook in the sense of undergoing the process of cooking.

• gotować = to cook something
• gotować się = to cook, to be cooking, to boil

So:

• Gotuję ziemniaki. = I am cooking the potatoes.
• Ziemniaki gotują się. = The potatoes are cooking.

The się here does not literally mean themselves in English. It is just part of the verb pattern.

Because ziemniaki is plural: potatoes.

In Polish, the verb agrees with the subject:

• ziemniak gotuje się = the potato is cooking
• ziemniaki gotują się = the potatoes are cooking

So gotują is the 3rd person plural form.

Because dłużej is an adverb, and here we are comparing the duration of an action: they cook longer.

• długi = long
• dłuższy = longer, as an adjective
• długo = long, for a long time
• dłużej = longer, for more time

Here the sentence is about how long the cooking takes, so Polish uses the adverb:

• gotują się dłużej = they cook for longer / they take longer to cook

If you used dłuższe, that would be an adjective describing a noun, for example dłuższe spodnie = longer trousers.

Niż means than in comparisons.

So:

• dłużej niż makaron = longer than pasta

It is the normal word used after a comparative:

• większy niż = bigger than
• szybciej niż = faster than
• dłużej niż = longer than

In Polish, makaron is normally treated as a mass/collective noun, so singular is very natural.

So:

• makaron = pasta
• lubię makaron = I like pasta

You can of course use plural in some contexts, but in a general food sense, singular is the standard choice.

Dlatego means therefore, for that reason, or that’s why.

In this sentence, it links the cause and the result:

• Ziemniaki gotują się dłużej niż makaron = Potatoes take longer to cook than pasta
• dlatego zaczynam je gotować wcześniej = that’s why I start cooking them earlier

So dlatego introduces the consequence.

Je refers back to ziemniaki.

So:

• zaczynam je gotować wcześniej = I start cooking them earlier

It is the accusative plural pronoun here, because the potatoes are the direct object of gotować.

Compare:

• Widzę ziemniaki. = I see the potatoes.
• Widzę je. = I see them.

Because in Polish, unstressed object pronouns like go, ją, je, ich often come before the infinitive or other verb forms quite naturally.

So:

• zaczynam je gotować = natural
• zaczynam gotować je = possible in some contexts, but less neutral and often more emphatic

Putting je before gotować is the most normal word order here.

After zaczynać (to begin / to start), Polish normally uses an imperfective infinitive.

That is why:

• zaczynam gotować = I start cooking

but not normally:

• zaczynam ugotować

This is because zaczynać focuses on the beginning of an action, and the infinitive after it is usually the ongoing/process form, not the completed-result form.

Here:

• gotować = to cook, as a process
• ugotować = to cook completely / to finish cooking

So zaczynam je gotować wcześniej is the natural choice.

Wcześniej means earlier.

It is an adverb, because it describes when the action starts:

• zaczynam ... wcześniej = I start ... earlier

Compare:

• wczesny = early, as an adjective
• wcześniej = earlier, as an adverb

So you need wcześniej, not an adjective form, because it modifies the verb zaczynam.

Yes. That is a very natural English translation.

Polish often uses gotują się where English may prefer:

• are cooking
• cook
• take longer to cook

So although the Polish structure is literally closer to Potatoes cook longer than pasta, the best English often becomes:

• Potatoes take longer to cook than pasta

This is a good example of how natural translation is often not word-for-word.

Because Polish normally separates clauses like these with a comma.

Here you have:

• clause 1: Ziemniaki gotują się dłużej niż makaron
• clause 2: dlatego zaczynam je gotować wcześniej

The comma helps mark the logical break: first the reason, then the consequence.

Polish punctuation is often stricter than English in this kind of sentence, so the comma is important.

Often, yes, but not always strictly only boil in the narrow English sense. In everyday Polish, gotować się can mean something like be cooking or be boiling, depending on context.

With foods like ziemniaki and makaron, the idea is usually that they are being cooked in boiling water.

So in this sentence, an English speaker can understand it as:

• the potatoes are boiling/cooking
• the potatoes take longer to cook

All of those are close enough in meaning here.