Hon missade sin tvättid i går, men grannen lät henne tvätta en timme senare.

Sin is a reflexive possessive. It refers back to the subject of the same clause, here Hon.

So in Hon missade sin tvättid, sin means her own.

If you said Hon missade hennes tvättid, it would usually mean She missed her laundry slot where her refers to some other woman, not the subject.

A useful rule:

• sin/sitt/sina = the subject’s own
• hans/hennes/deras = someone else’s, or used when you want to be explicit

In Swedish, a possessive normally replaces the article.

So:

• sin tvättid = her laundry slot
• not sin en tvättid or anything like that

This is similar to English:

• her book, not her a book

Tvättid is a compound noun:

• tvätt = washing / laundry
• tid = time

Together, tvättid means a laundry time slot.

In Sweden, this often refers to a booked time in a shared laundry room in an apartment building. So it is not just “time to do laundry” in a general sense; it often means a specific reserved slot.

Missade is the past tense (preterite) of missa.

• att missa = to miss
• missade = missed

So Hon missade sin tvättid uses a normal completed past action: She missed her laundry slot.

Mostly yes. In this sentence, missa means something like:

• fail to make
• fail to catch
• be too late for
• not show up in time for

So missade sin tvättid means she did not use her reserved laundry slot at the proper time.

Here i går is the adverb meaning yesterday.

You do not need to analyze it literally when learning the sentence; just treat i går as a fixed time expression meaning yesterday.

In this sentence it appears after sin tvättid:

• Hon missade sin tvättid i går

That placement is very natural in Swedish.

Grannen is the definite form and means the neighbor.

Swedish often uses the definite form when the person is identifiable from the situation or context. Here it sounds like a specific neighbor is meant, not just any neighbor.

Compare:

• en granne = a neighbor
• grannen = the neighbor

The definite ending -en is attached to the noun itself.

Lät is the past tense of låta.

In this sentence, låta means let or allow.

So:

• låta någon göra något = to let someone do something
• grannen lät henne tvätta = the neighbor let her wash / do laundry

Be careful: låta can have other meanings in other contexts, but here it clearly means allow.

Because henne is the object form of hon.

• hon = she
• henne = her

In grannen lät henne tvätta, the neighbor is doing the letting, and henne is the person being allowed to do something. That makes henne the object.

Compare:

• Hon tvättar. = She is washing.
• Grannen lät henne tvätta. = The neighbor let her wash.

After låta, Swedish uses the bare infinitive, meaning the infinitive without att.

So you say:

• låta någon tvätta
• not låta någon att tvätta

This is similar to English:

• let someone wash
• not let someone to wash

So lät henne tvätta is the expected structure.

It follows a very common pattern:

låta + object + infinitive

Here:

• lät = let / allowed
• henne = her
• tvätta = wash / do laundry

So the structure is:

• grannen lät henne tvätta
• literally: the neighbor let her wash

This is a very useful Swedish pattern to learn.

En timme senare means an hour later.

Breakdown:

• en timme = an hour
• senare = later

So the idea is that the neighbor allowed her to do the laundry one hour after the original time.

Swedish often uses senare this way after a time expression:

• två dagar senare = two days later
• lite senare = a little later
• en timme senare = an hour later

Because after men (but), Swedish starts a new main clause.

In a main clause, Swedish normally keeps the finite verb in second position. Here the subject comes first:

• grannen = subject
• lät = finite verb

So:

• men grannen lät henne tvätta...

That is the normal, straightforward order.

If another element were placed first for emphasis, the verb would still stay in second position:

• Men en timme senare lät grannen henne tvätta.

That is also grammatical, but it shifts the emphasis.

They are related words built from the same root idea:

• tvätta = to wash / do laundry
• tvätt = washing / laundry
• tvättid = laundry time slot

Swedish forms many nouns this way. So this sentence is a nice example of how related verb and noun forms appear together.

Yes. In everyday Swedish, tvätta can literally mean wash, but in a context like tvättid it often clearly means do the laundry.

So lät henne tvätta is naturally understood as let her do her laundry, not just let her wash in some vague sense.

Context does that work for you.