Jeg tager først ukrudtet op, når jorden er fugtig.

Because tage op is a particle verb in Danish. In this sentence, tager is the finite verb, and op is the particle.

In a main clause, Danish usually puts the finite verb early in the sentence, so the verb and particle often get split:

Jeg tager ukrudtet op.

This is very similar to English verbs like pick up or pull up.

In gardening, tage ... op means pull up / remove from the ground.

Here it means to pull up the weeds or remove the weeds from the soil, usually by the roots.

Even though tage often means take, Danish uses it in many fixed expressions. So you should learn tage op here as a unit of meaning, not translate it word-for-word as take up.

In this context:

• tage ukrudtet op = weed / pull up the weeds

Because ukrudtet is the definite form: the weeds / the weed growth.

Danish usually adds the definite article to the end of the noun:

• ukrudt = weed / weeds / weed growth
• ukrudtet = the weeds / the weed growth

A native English speaker often expects a separate word like the, but Danish often uses an ending instead.

Also, ukrudt is often treated as a collective or mass noun, not just a regular plural noun. So ukrudtet can mean the weeds as a general patch of weeds, not necessarily a counted plural in the English sense.

Often, it is best understood as a mass noun or collective noun.

So in many gardening contexts:

• ukrudt = weeds in general
• ukrudtet = the weeds already being talked about

This can feel unusual to English speakers, because English normally uses the plural weeds. Danish often treats it more like vegetation or weed growth.

You can sometimes find countable uses too, but in everyday gardening language, the collective meaning is very common.

For the same reason as ukrudtet: jorden is the definite form.

• jord = soil / earth / ground
• jorden = the soil / the ground

Here the speaker means a specific soil context, probably the soil in the garden bed, so jorden is natural.

It is also worth noting that jord can mean:

• soil
• earth
• ground

In this sentence, soil is the best fit.

That is a very common question. Here først means something more like:

• not until
• only when

So:

Jeg tager først ukrudtet op, når jorden er fugtig.

means the speaker waits until the soil is moist before pulling up the weeds.

This is a very common Danish pattern:

• først ..., når ... = not until / only when

So it is not really about doing something first in a sequence. It is about a time condition.

Because først is modifying the action tager ... op as a whole.

The sentence structure makes først apply to the timing of the action:

• Jeg tager først ukrudtet op ...
  = I only pull up the weeds once / when ...

Its placement is natural in Danish and helps create the not until meaning together with når.

Because når is the normal choice for a general time condition or something that happens whenever / when a condition is met.

Here the speaker is expressing a general rule or habit:

• I pull up the weeds when the soil is moist

Compare:

• når = when / whenever
• hvis = if
• da = when, but usually about a specific past occasion

So:

• Når jorden er fugtig = When/whenever the soil is moist
• Hvis jorden er fugtig would sound more like If the soil is moist
• Da jorden var fugtig would refer to a specific past situation: When the soil was moist

Because når jorden er fugtig is a subordinate clause.

In Danish:

• main clauses usually follow verb-second word order
• subordinate clauses usually do not

So:

• Main clause: Jeg tager først ukrudtet op
• Subordinate clause: når jorden er fugtig

In the subordinate clause, the subject jorden comes before the verb er.

You can also move the subordinate clause to the front:

Når jorden er fugtig, tager jeg først ukrudtet op.

Now the main clause shows inversion:

• tager jeg instead of jeg tager

That is classic Danish word order.

Because fugtig means moist / damp, while våd means wet.

For soil, fugtig is more natural if you mean:

• slightly moist
• soft enough to work with
• not dry

That fits gardening very well, because weeds are often easier to pull up when the soil is moist, not necessarily soaking wet.

So:

• fugtig jord = moist soil
• våd jord = wet soil

Danish very often uses the present tense for:

• habits
• general truths
• routines
• near-future actions

So Jeg tager først ukrudtet op, når jorden er fugtig can mean a general habit or rule:

I only pull up the weeds when the soil is moist.

You do not need a separate future form here.

If you said jeg vil tage, that could suggest:

• willingness
• intention
• a specific future plan

But the plain present tager is the most natural choice for a general statement like this.

Yes. That version is perfectly natural.

It has the same basic meaning, but the emphasis shifts slightly:

• Jeg tager først ukrudtet op, når jorden er fugtig
  starts with what I do
• Når jorden er fugtig, tager jeg først ukrudtet op
  starts with the condition

So the second version highlights when the action happens.

This is a very common and useful word-order variation in Danish.