Jag måste ta bort ogräset i dag.

A natural breakdown is:

• Jag = I
• måste = must / have to
• ta bort = remove / take away
• ogräset = the weeds / the weed growth
• i dag = today

So the structure is:

• subject: Jag
• finite verb: måste
• infinitive verb phrase: ta bort
• object: ogräset
• time expression: i dag

A very literal gloss would be something like:

I must take away the weeds today.

Because måste is a modal verb, and in Swedish modal verbs are normally followed by the infinitive without att.

So you get:

• Jag måste ta bort ogräset
• Jag kan ta bort ogräset
• Jag vill ta bort ogräset

Not:

• Jag måste att ta bort ogräset

This is similar to English, where we say I must remove it, not I must to remove it.

Ta bort is a very common Swedish particle verb. You should learn it as one unit.

• ta = take
• bort = away

Together, ta bort often means:

• remove
• take away
• get rid of

So in this sentence, ta bort ogräset means remove the weeds.

This is one of those places where Swedish uses a very ordinary everyday expression, even if English might choose a more specific verb like weed, pull up, or clear away.

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

The base noun is:

• ett ogräs = a weed

The form ogräset is:

• the weed
• or more naturally in this kind of context, the weeds / the weed growth

In Swedish, nouns like this can be used a bit more collectively than in English. So even though English often says the weeds in the plural, Swedish can naturally use ogräset to mean the unwanted weed growth in a garden, flowerbed, etc.

So ta bort ogräset sounds like removing the weeds in general from some area.

It is best to think of ta bort as one meaning-unit, even though it is written as two words.

In ta bort ogräset:

• ta bort = the verb expression
• ogräset = the object

So the object comes after the whole verb expression.

This is normal for Swedish particle verbs. Similar examples:

• slå av lampan = turn off the lamp
• skriva upp numret = write down the number
• ställa in mötet = cancel the meeting

So yes: grammatically it is a verb plus a particle, but in practice you should learn ta bort as a set expression.

Yes. Both i dag and idag are used and understood as today.

The two-word spelling i dag is often seen as a bit more traditional, while idag is also very common in modern Swedish.

So these mean the same thing:

• Jag måste ta bort ogräset i dag
• Jag måste ta bort ogräset idag

You can treat them as spelling variants.

Yes, i dag can come first.

In the original sentence, the order is:

• Jag = subject
• måste = finite verb
• the rest follows

This is the normal order for a simple statement.

But Swedish is a V2 language, which means the finite verb usually stays in the second position in main clauses. So if you move i dag to the front, the verb still has to stay second:

• I dag måste jag ta bort ogräset

Notice that jag moves after måste.

So:

• Jag måste ta bort ogräset i dag = neutral statement
• I dag måste jag ta bort ogräset = same meaning, but with today emphasized a bit more

No. In Swedish, verbs do not change for person the way they do in many other languages.

So you get:

• jag måste
• du måste
• han måste
• vi måste
• de måste

The form stays the same.

This is true for Swedish verbs in general in the present tense:

• jag tar
• du tar
• hon tar
• vi tar

So once you learn the present-tense form, you can use it with all subjects.

Yes. Rensa ogräs is also very common and often feels especially natural when talking about gardening.

For example:

• Jag måste rensa ogräs i dag = I have to weed today
• Jag måste ta bort ogräset i dag = I have to remove the weeds today

The sentence you gave is absolutely understandable and natural, but rensa ogräs is a very common gardening expression.

A rough difference is:

• ta bort ogräset focuses on removing it
• rensa ogräs focuses on weeding / clearing weeds

Both are useful.

Time expressions are quite flexible in Swedish, but putting i dag at the end is very normal.

So all of these are possible, depending on emphasis:

• Jag måste ta bort ogräset i dag
• I dag måste jag ta bort ogräset
• Jag måste i dag ta bort ogräset (possible, but less neutral)

The version with i dag at the end is a very natural default word order in everyday Swedish. It simply adds the time information after the main action.