Vi måste ta bort ogräset om gräsmattan ska se fin ut i helgen.

Because ta bort is a fixed verb phrase meaning remove, take away, or get rid of.

• ta = take
• ta bort = remove / take away

So:

• ta ogräset would sound incomplete or odd here
• ta bort ogräset clearly means remove the weeds

This works a lot like a phrasal verb in English.

In Swedish, particles like bort, upp, ut, and igen often combine with verbs to form particle verbs.

So:

• ta bort = remove
• se ut = look / appear

In infinitive form, the verb and particle usually stay together:

• måste ta bort ogräset

But in some sentence patterns, the particle can be separated. For example:

• Vi tar bort ogräset.
• Vi tar ogräset bort. ← generally not natural here

So in this sentence, ta bort is the natural combination.

Ogräs means weed or weeds, and ogräset is the definite form: the weeds or the weed.

Swedish often uses the definite form when talking about something specific and already understood from the situation. Here, it means the weeds in the lawn that need removing.

This is very common in Swedish. Instead of a separate word like the, Swedish usually adds the definite ending to the noun:

• gräsmatta = lawn
• gräsmattan = the lawn
• ogräs = weed(s)
• ogräset = the weed / the weeds

With words like ogräs, Swedish often treats them as a mass or collective noun, so ogräset can mean the weeds in a general practical sense.

Because the sentence is talking about a specific lawn, not just any lawn.

• en gräsmatta = a lawn
• gräsmattan = the lawn

In context, the speaker clearly means a particular lawn, probably their own or one already known to both speakers. So Swedish uses the definite form.

Because Swedish usually puts the definite article at the end of the noun instead of using a separate word before it.

Examples:

• bok = book

• boken = the book

• gräsmatta = lawn

• gräsmattan = the lawn

• ogräs = weed(s)
• ogräset = the weed(s)

So the meaning of the is built into the noun itself.

Here om means if.

So:

• om gräsmattan ska se fin ut = if the lawn is going to look nice / if the lawn is to look nice

This is the normal word for if in conditional sentences.

Be aware that om can also mean other things in other contexts, such as about, but here it clearly means if.

Because after a subordinating conjunction like om, Swedish uses subordinate clause word order.

That means the subject comes before sentence adverbs and the finite verb does not move to second position the way it does in a main clause.

So you get:

• om gräsmattan ska se fin ut

not:

• om ska gräsmattan se fin ut

Compare:

• Main clause: Gräsmattan ska se fin ut.
• Subordinate clause: ... om gräsmattan ska se fin ut.

This is one of the biggest word-order differences learners notice in Swedish.

Not really in modern English terms. In this sentence, ska is closer to:

• is going to
• is supposed to
• is to
• sometimes simply is to be expected to

So:

• om gräsmattan ska se fin ut i helgen

means something like:

• if the lawn is going to look nice this weekend
• if the lawn is to look nice this weekend

It is not the same as must. The word for must here is måste.

Because they do different jobs.

• måste = must / have to
• ska = is going to / is to / should in the sense of intended result

So the structure is:

• Vi måste... = We must...
• ... om gräsmattan ska... = ...if the lawn is to...

In other words:

We have to do X if Y is going to happen.

That is a very natural Swedish structure.

Both are possible, but they mean slightly different things.

• se fin ut = look nice
• vara fin = be nice / be pretty / be in nice condition

In this sentence, se fin ut focuses on appearance, which fits a lawn very well.

• Gräsmattan ska se fin ut = The lawn should look nice

If you said vara fin, it would sound a bit more like describing its general state rather than specifically how it looks.

Se ut is a very common particle verb meaning look, appear, or look like, depending on context.

Examples:

• Hon ser trött ut. = She looks tired.
• Det ser bra ut. = It looks good.
• Hur ser huset ut? = What does the house look like?

So in your sentence:

• gräsmattan ska se fin ut = the lawn is going to look nice

This is one of the most useful everyday verb phrases in Swedish.

Because fin is a predicative adjective agreeing with gräsmattan, which is a common gender singular noun.

• en gräsmatta → common gender
• therefore: fin

Compare:

• Gräsmattan är fin. = The lawn is nice.
• Huset är fint. = The house is nice.
  (hus is neuter, so fint)

So the adjective changes depending on the noun it refers to.

Måste is present tense, but like English must, it often refers to a present necessity about the future.

So:

• Vi måste ta bort ogräset = We must / have to remove the weeds

It means the necessity exists now, even though the action may happen later.

Swedish does this very naturally with present tense forms.

I helgen means this weekend or at the weekend, depending on context.

It is a fixed time expression:

• i dag = today
• i morgon = tomorrow
• i veckan = this week
• i helgen = this weekend

In this sentence, it refers to the coming weekend.

Yes. A very natural alternative is:

Om gräsmattan ska se fin ut i helgen, måste vi ta bort ogräset.

This means the same thing.

Notice what happens in the main clause after the subordinate clause comes first:

• måste vi
• not vi måste

That is because Swedish main clauses follow the V2 rule: the finite verb comes in second position.

So both are correct:

• Vi måste ta bort ogräset om gräsmattan ska se fin ut i helgen.
• Om gräsmattan ska se fin ut i helgen, måste vi ta bort ogräset.

It is a bit tricky, because ogräs often behaves like a collective or mass noun in Swedish.

Depending on context, it can mean:

• weed
• weeds
• weed growth

So ogräset here is best understood as the weeds or the weed growth in the lawn.

This is why a direct one-to-one mapping with English singular/plural is not always perfect.

Sometimes, but the meaning changes.

• måste = must / have to
• behöver = need to

So:

• Vi måste ta bort ogräset = We must remove the weeds
• Vi behöver ta bort ogräset = We need to remove the weeds

Both are possible, but måste sounds stronger and more urgent. In your sentence, that stronger feeling fits well.

It breaks down like this:

• Vi = we
• måste = must / have to
• ta bort = remove
• ogräset = the weeds
• om = if
• gräsmattan = the lawn
• ska = is going to / is to
• se ... ut = look
• fin = nice
• i helgen = this weekend

So structurally:

Main clause:
Vi måste ta bort ogräset
= We must remove the weeds

Subordinate clause:
om gräsmattan ska se fin ut i helgen
= if the lawn is going to look nice this weekend

That makes it a very good example of:

• a modal verb (måste)
• a particle verb (ta bort)
• a subordinate clause with om
• another particle verb (se ut)
• adjective agreement (fin)