Den nya jorden är lättare än den gamla, så jag byter kruka i eftermiddag.

Because Swedish uses double definiteness when a noun is definite and has an adjective before it.

So in den nya jorden:

• den = the definite article for a common-gender noun
• nya = the adjective in its definite form
• jorden = the noun in its definite form

This is very common in Swedish:

• den röda bilen = the red car
• det stora huset = the big house

So den nya jorden literally has both a definite article and a definite noun form. That is normal Swedish grammar.

Because jord is a common-gender noun in Swedish, not a neuter noun.

Swedish nouns usually belong to one of two genders:

• en-words (common gender)
• ett-words (neuter)

jord is an en-word, so in definite phrases you use den:

• en jord
• den jorden

If it were a neuter noun, you would use det instead.

Because adjectives take a special form when they come before a definite noun.

Compare:

• en ny jord = a new soil
• den nya jorden = the new soil

And:

• en gammal jord = an old soil
• den gamla jorden = the old soil

In definite noun phrases, adjectives usually take -a:

• den stora bilen
• det lilla huset
• de röda blommorna

So nya and gamla are the expected definite forms here.

Yes, jorden can mean the earth, but jord also means soil. Swedish uses the same word for both ideas, and context tells you which meaning is meant.

In a sentence about changing a pot, jorden is naturally understood as the soil or the potting soil, not the planet.

This kind of overlap is normal in many languages.

Lättare is the comparative form of lätt (light, in weight).
Än means than.

So:

• lätt = light
• lättare = lighter
• än = than

Together:

• lättare än den gamla = lighter than the old one

This is the standard Swedish way to make comparisons:

• starkare än = stronger than
• dyrare än = more expensive than
• snabbare än = faster than

Because Swedish often leaves the noun understood when it is obvious from context.

In lättare än den gamla, the noun jorden is not repeated, but it is understood:

• den gamla = the old one / the old soil

English often uses one in this situation, but Swedish usually does not need an extra word.

For example:

• Jag tar den röda. = I’ll take the red one.
• Den nya är bättre än den gamla. = The new one is better than the old one.

Here så means so, therefore, or as a result.

It connects the two parts of the sentence:

• Den nya jorden är lättare än den gamla
• så jag byter kruka i eftermiddag

So the idea is: one fact leads to the decision or action in the next clause.

In everyday Swedish, så is very common as a connector.

Because Swedish often uses the present tense for planned or intended future actions, especially when there is a time expression.

So:

• jag byter kruka i eftermiddag

means something like:

• I’m changing the pot this afternoon
• I’ll change the pot this afternoon

This is very natural in Swedish. Other examples:

• Jag åker imorgon. = I’m leaving tomorrow.
• Vi ses ikväll. = We’ll see each other tonight.

The time phrase makes the future meaning clear.

Because Swedish often leaves the object indefinite and article-less in expressions where the activity itself is the main idea.

Here, byter kruka is understood as changing pot / repotting / switching to a different pot. The focus is on the action, not on identifying one specific pot.

Compare the possible shades of meaning:

• jag byter kruka = I’m changing pot / repotting
• jag byter en kruka = I’m changing a pot (less natural in this context)
• jag byter krukan = I’m changing the pot (a specific known pot)

So the form in the sentence is very natural for this kind of practical action.

I eftermiddag means this afternoon.

This expression is used for a specific upcoming afternoon, usually the same day.

Compare:

• i eftermiddag = this afternoon
• på eftermiddagen = in the afternoon / during the afternoon

So in this sentence, i eftermiddag fits because the speaker means a particular planned time: later today.

The word order is very normal Swedish main-clause word order:

• Den nya jorden = subject
• är = verb
• lättare än den gamla = comparison
• så = connector
• jag = subject
• byter = verb
• kruka = object
• i eftermiddag = time expression

A useful thing to notice is that Swedish likes the verb early in the clause. In ordinary main clauses, the finite verb is typically in the second position.

That is exactly what you see here:

• Den nya jorden är ...
• så jag byter ...

So the sentence is a good example of straightforward Swedish clause structure.