Hun tager ukrudtet op, mens jeg vander planterne.

In Danish, the is very often attached to the end of the noun instead of being a separate word.

• et ukrudt = a weed / weed

• ukrudtet = the weed / the weeds as a mass or collective idea

• en plante = a plant
• planterne = the plants

So in this sentence, Danish expresses definiteness with endings:

• -et in ukrudtet
• -ne in planterne

That is one of the biggest differences from English noun structure.

They show different grammatical forms.

• ukrudtet is definite singular of et ukrudt
• planterne is definite plural of planter

A quick breakdown:

• et ukrudt → ukrudtet
• en plante → planten
• planter → planterne

So:

• -et often marks definite singular for a neuter noun
• -ne marks definite plural

Usually ukrudt is used as a kind of mass noun or collective noun, much like English weed or weeds when talking about unwanted growth in a garden in general.

So ukrudtet here is best understood as:

• the weeds
• the weed growth
• the unwanted plants in the soil

Even though the form is singular, it often refers to the general patch of weeds, not just one single weed.

Because tage op is a separable verb expression. In this sentence, the finite verb goes earlier in the clause, and the particle op stays later.

So:

• infinitive: at tage ukrudtet op
• present tense main clause: Hun tager ukrudtet op

This is similar to English phrasal verbs like pick up or pull up, where the second part can be separated.

Here tage op means something like:

• pull up
• take up out of the ground

So the sentence structure is normal Danish.

Because they are in the present tense.

In Danish, the present tense is usually made by adding -r to the infinitive or stem:

• tage → tager
• vande → vander

Unlike English, Danish verbs do not change depending on the subject:

• jeg vander
• du vander
• hun vander
• vi vander

The verb form stays the same for all persons.

Because mens introduces a subordinate clause, and subordinate clauses in Danish normally do not use main-clause V2 word order.

So after mens, the usual order is:

• subject + verb

That is why we get:

• mens jeg vander planterne

not:

• mens vander jeg planterne

Compare:

• Main clause: Jeg vander planterne
• Subordinate clause: mens jeg vander planterne

If you put the mens clause first, then the following main clause shows inversion:

• Mens jeg vander planterne, tager hun ukrudtet op.

Notice tager hun, not hun tager, because the main clause is now coming after a fronted clause.

Here mens means while, showing that two actions are happening at the same time.

So the idea is:

• she is pulling up the weeds
• at the same time, I am watering the plants

In other contexts, mens can also mean something closer to whereas, but in this sentence it clearly has a time meaning: while.

You will often see a comma before a subordinate clause like this:

• Hun tager ukrudtet op, mens jeg vander planterne.

That is a very normal written form.

However, Danish comma rules allow different systems, and in some styles the comma before mens may be omitted:

• Hun tager ukrudtet op mens jeg vander planterne.

For a learner, using the comma here is a safe and natural choice.

Yes. A very common verb is luge, which means to weed.

So you could also hear:

• Hun luger, mens jeg vander planterne.

The difference is:

• luge = to weed in general
• tage ukrudtet op = to pull the weeds up, with a slightly more literal feel

So the original sentence is correct and natural, but luge is also very common in gardening contexts.