När hon hade mätt väggen med måttbandet såg hon att tavlan satt för lågt.

Hade mätt is the pluperfect (past perfect): had measured.

It shows that this action happened before the next past action:

• hade mätt = had measured
• såg = saw

So the sequence is:

1. she measured the wall
2. then she saw that the painting was too low

If you said När hon mätte väggen ..., it would mean when she measured the wall, but it would not emphasize as clearly that the measuring was already completed before she noticed the problem.

This is because Swedish uses verb-second word order in main clauses.

The sentence begins with the subordinate clause:

• När hon hade mätt väggen med måttbandet = when she had measured the wall with the measuring tape

After that comes the main clause, and in a Swedish main clause the finite verb must come in the second position:

• såg hon
• not hon såg

So the structure is:

• first element: När hon hade mätt väggen med måttbandet
• second element: såg
• then subject: hon

This is very common in Swedish.

Compare:

• Hon såg att tavlan satt för lågt.
• När hon hade mätt väggen med måttbandet såg hon att tavlan satt för lågt.

Here När means when.

It introduces a subordinate clause that gives the time background for the main event:

• När hon hade mätt väggen ... = When she had measured the wall ...

A learner should notice that this is not a question word here. It is acting as a conjunction introducing a time clause.

In Swedish, väggen means the wall and måttbandet means the measuring tape.

The endings show definiteness:

• vägg = wall

• väggen = the wall

• måttband = measuring tape
• måttbandet = the measuring tape

Swedish often puts the definite article as a suffix on the noun, rather than using a separate word like the.

In this sentence, it sounds natural because we are talking about a specific wall and a specific measuring tape.

It means with the measuring tape.

Here med means with, and it shows the instrument used to do the measuring.

So:

• mäta något med något = to measure something with something

Examples:

• Hon mätte bordet med en linjal. = She measured the table with a ruler.
• Han skar brödet med en kniv. = He cut the bread with a knife.

This is a very common Swedish usage.

Sitta can be used for things that are positioned, attached, mounted, or located in a particular place, especially when their placement matters.

So:

• Tavlan satt för lågt = The painting was positioned too low

This often sounds more natural in Swedish than just var.

Rough differences:

• var = was; very general
• hängde = hung; focuses more on the fact that it was hanging
• satt = was sitting/placed/attached; focuses on placement or position

For a painting on a wall, satt is very idiomatic if you are talking about whether it is placed correctly.

Because lågt is being used like an adverb here.

In Swedish, adjectives often take the -t form when they describe how something is positioned, placed, done, etc.

So:

• låg = low
• lågt = low / in a low position

In tavlan satt för lågt, the meaning is basically:

• the painting was sitting/positioned too low

Compare:

• ett lågt bord = a low table
  Here lågt is an adjective agreeing with the neuter noun bord
• bordet står lågt = the table stands low
  Here lågt works adverbially

In your sentence, lågt describes the placement, not the noun directly.

Here för means too.

So:

• för lågt = too low

This is a very common pattern:

• för högt = too high
• för stort = too big
• för dyrt = too expensive
• för sent = too late

It means more than is acceptable or appropriate.

Because att here means that and introduces a clause after såg:

• såg hon att tavlan satt för lågt
• she saw that the painting was too low

Swedish often uses att in this kind of sentence, just like English uses that.

The clause after att is a subordinate clause:

• att tavlan satt för lågt

Yes. After att, Swedish uses subordinate clause word order, which is different from main-clause word order in some situations.

Here you get:

• att tavlan satt för lågt

The subject comes before the verb:

• tavlan = subject
• satt = verb

That is normal for a subordinate clause.

A very important thing for learners is that Swedish main clauses often show verb-second order, but subordinate clauses do not work the same way.

Yes, tavlan hängde för lågt is also possible.

That would mean:

• the painting hung too low

This version focuses more directly on the fact that the painting was hanging on the wall.

By contrast:

• tavlan satt för lågt focuses more on the painting’s placed position

Both can be natural, but satt is very idiomatic when talking about the placement of attached objects like pictures, signs, handles, buttons, and similar things.

Because mätt is the supine/past participle form used with ha in the verb mäta.

The key forms are:

• mäta = to measure
• mäter = measures / is measuring
• mätte = measured
• mätt = measured

So:

• hon hade mätt = she had measured

This is just one of the principal forms you need to learn with the verb.

Yes, it is natural and idiomatic.

It contains several very typical Swedish features:

• a time clause first: När hon hade mätt ...
• verb-second in the main clause: såg hon
• a common verb choice for placement: tavlan satt
• an adverbial expression of excess: för lågt

So it is a very useful sentence for learning normal Swedish grammar and style.