Jag måste tvätta klart innan gästerna kommer.

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

So:

• Jag måste tvätta = I have to wash
• Jag kan tvätta = I can wash
• Jag vill tvätta = I want to wash

Using att after måste would be incorrect in standard Swedish.

Tvätta klart means to finish washing or to get the washing done completely.

Here, klart adds the idea of completion. It is very common in Swedish to add words like this after a verb to show that something is finished:

• läsa klart = finish reading
• skriva klart = finish writing
• äta klart = finish eating
• tvätta klart = finish washing

So tvätta by itself just means wash, but tvätta klart means wash everything that needs to be washed and finish the task.

Swedish often leaves out the object when it is obvious from context.

So Jag måste tvätta klart can mean things like:

• I have to finish the washing
• I have to finish doing the laundry
• I have to finish washing up

The exact object is not stated, because the speaker assumes the listener already knows what task is meant.

English does this too sometimes, though Swedish is often a bit more comfortable leaving it unsaid.

In this kind of expression, klart is the fixed form normally used with the verb to mean finished / completely done.

It comes from the adjective klar:

• en klar sak
• ett klart besked

But in expressions like läsa klart, äta klart, tvätta klart, the form klart functions almost like an adverb or completion marker. Learners usually do best to memorize these as whole expressions.

So here, do not think too much of klart as describing a noun. Think of tvätta klart as one unit meaning finish washing.

Gästerna means the guests.

Breakdown:

• gäst = guest
• gäster = guests
• gästerna = the guests

Swedish usually puts the definite article at the end of the noun as a suffix. So instead of a separate word like the, Swedish often uses an ending:

• bok = book
• boken = the book
• böcker = books
• böckerna = the books

So gästerna kommer literally means the guests are coming / the guests come.

Because Swedish very often uses the present tense for future meaning when the future action is clear from context.

Here, innan and the situation make it obvious that this is about the future:

• innan gästerna kommer = before the guests arrive

This is very natural Swedish. English also does something similar in time clauses:

• before the guests arrive, not usually before the guests will arrive

So Swedish kommer here is present tense in form, but future in meaning.

Innan means before and introduces a time clause.

In this sentence:

• innan gästerna kommer = before the guests arrive

A useful rule:

• innan is used before a clause with a subject and verb
• före is more often used before a noun phrase

Compare:

• innan gästerna kommer = before the guests arrive
• före middagen = before dinner

So innan is the natural choice here because gästerna kommer is a full clause.

Because innan gästerna kommer is a subordinate clause, and in Swedish subordinate clauses normally keep the order subject + verb.

So:

• gästerna kommer = the guests arrive / are coming

That is normal subordinate-clause word order.

You may be thinking of inversion, where Swedish sometimes puts the verb before the subject. That happens in main clauses after certain sentence openings, but not here inside the innan clause.

Yes. You can say:

• Innan gästerna kommer måste jag tvätta klart.

That is also correct.

But when the subordinate clause comes first, the main clause must follow normal Swedish V2 word order, so the verb comes before the subject:

• Innan gästerna kommer måste jag...
• not Innan gästerna kommer jag måste...

So the original sentence and the rearranged version are both natural:

• Jag måste tvätta klart innan gästerna kommer.
• Innan gästerna kommer måste jag tvätta klart.

Yes, in many contexts you can. Both can mean finish washing.

• tvätta klart
• tvätta färdigt

Both are natural, though klart is extremely common in everyday Swedish with many verbs. Sometimes choice depends a little on style, dialect, or habit, but for a learner, both are good to recognize.

A natural rhythm would usually stress the important content words:

Jag måste TVÄTTA klart innan GÄSTERNA kommer.

A few pronunciation notes:

• jag is often pronounced more like ya in casual speech
• måste is roughly MOS-teh
• tvätta has a tv cluster that many learners find tricky; the stress is on the first syllable
• gästerna has the vowel ä, and the stress is on the first syllable

In careful speech, the sentence is clear and steady, but in everyday speech some words become shorter and lighter, especially jag and innan.