Om tvättkorgen är full, tvättar jag hellre ikväll än i morgon bitti.

Tvättkorgen is a Swedish compound noun.

• tvätt = washing / laundry
• korg = basket
• -en = the definite ending

So tvättkorgen means the laundry basket.

Swedish makes compounds much more freely than English does, so where English often writes two words, Swedish often writes one.

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

• en tvättkorg = a laundry basket
• tvättkorgen = the laundry basket

In context, this usually means the basket in the home that both speaker and listener understand.

The adjective agrees with the noun.

Tvättkorg is an en-word and singular, so the predicative adjective is full:

• Korgen är full. = The basket is full.

Compare:

• Huset är fullt. = The house is full.
  (hus is a ett-word)
• Korgarna är fulla. = The baskets are full.
  (plural)

So full is correct because tvättkorgen is singular and common gender.

Here om means if and introduces a condition:

• Om tvättkorgen är full ... = If the laundry basket is full ...

That fits because the speaker is talking about a possible situation.

När means when, and it usually sounds more definite or expected:

• När tvättkorgen är full ... = When the laundry basket is full ...

So:

• om = if this happens
• när = when this happens

Both can be possible in some contexts, but om is the natural choice for a condition like this.

Because Swedish main clauses follow the V2 rule: the finite verb normally comes in the second position.

In this sentence, the whole om-clause comes first:

• Om tvättkorgen är full = first position

So the finite verb of the main clause must come next:

• tvättar jag

That is why you get:

• Om tvättkorgen är full, tvättar jag ...

and not:

• Om tvättkorgen är full, jag tvättar ...

Because Swedish often uses the simple present for future meaning when the context is clear.

Here the time expressions make the future obvious:

• ikväll = tonight
• i morgon bitti = tomorrow morning

So tvättar jag can naturally mean I’ll do the laundry.

You could also say ska tvätta to make the future more explicit, but the present tense is very normal here.

Hellre ... än ... means rather ... than ....

It is used to compare two options:

• hellre ikväll än i morgon bitti
  = rather tonight than tomorrow morning

So the speaker is saying that, if the basket is full, tonight is the preferred option.

A simple pattern is:

• Jag gör hellre X än Y.
  = I’d rather do X than Y.

• ikväll = tonight / this evening
• i morgon bitti = tomorrow morning, often with a slight sense of early tomorrow morning

As for spelling:

• ikväll can also be written i kväll
• i morgon can also be written imorgon

So you may also see:

• imorgon bitti
• i kväll

These are spelling variants. The meaning stays the same.

Bitti is a common word in the expression i morgon bitti, meaning tomorrow morning.

It often feels a bit more specific than just i morgon, because it points to the morning part of tomorrow, often somewhat early.

Compare:

• i morgon = tomorrow
• i morgon bitti = tomorrow morning

So än i morgon bitti clearly contrasts tonight with tomorrow morning.

Yes. In everyday Swedish, tvätta can mean do the laundry when the context makes that clear.

So:

• tvättar jag here means something like I do the laundry / I wash clothes

The object is understood from the situation, especially because tvättkorgen already makes the laundry context obvious.

Not strictly.

In Swedish, a comma after an introductory subordinate clause is often optional, especially in a short sentence. Many writers still use it because it makes the sentence easier to read.

So both of these are possible:

• Om tvättkorgen är full, tvättar jag hellre ikväll än i morgon bitti.
• Om tvättkorgen är full tvättar jag hellre ikväll än i morgon bitti.

The version with the comma is very natural and clear.

That depends on whether it is in the subordinate clause or the main clause.

In a subordinate clause, inte usually comes before the finite verb:

• Om tvättkorgen inte är full ...

In a main clause, inte usually comes after the finite verb:

• ... tvättar jag inte ikväll.

So this sentence is a good example of the difference between Swedish main-clause and subordinate-clause word order.