Jag skulle precis mäta väggen när Sara ringde.

In this sentence, skulle does not mean should.

Here, skulle + infinitive means something like:

• was going to
• was about to
• intended to

So Jag skulle precis mäta väggen means the speaker was right on the point of measuring the wall.

This is a very common use of skulle in Swedish when talking about a future action seen from a past point in time.

Because skulle is a helping verb, and after skulle you use the infinitive form of the main verb.

So:

• skulle mäta = was going to measure
• not skulle mätte

This works like other modal-style verb combinations in Swedish:

• Jag kan mäta = I can measure
• Jag vill mäta = I want to measure
• Jag skulle mäta = I was going to measure

The finite verb is skulle, and the main verb stays in the infinitive: mäta.

Here precis means just or exactly at that moment.

So skulle precis mäta means:

• was just about to measure
• was just going to measure

It adds the idea that the action was very close to happening when Sara called.

Compare:

• Jag skulle mäta väggen = I was going to measure the wall
• Jag skulle precis mäta väggen = I was just about to measure the wall

That is a very natural position for adverbs like precis in Swedish.

In a main clause, the usual pattern is:

subject + finite verb + adverb + infinitive

So:

• Jag skulle precis mäta väggen

This is similar to:

• Jag har redan ätit
• Jag kan nog komma
• Jag ville verkligen hjälpa

You may sometimes see slightly different placement in other structures, but in this sentence skulle precis mäta is the normal and natural order.

Väggen means the wall, while en vägg means a wall.

The definite form väggen suggests that the speaker means a specific wall that is already known from the situation or context, for example:

• the wall in the room
• the wall they were talking about
• the wall the speaker was planning to work on

If you said mäta en vägg, it would sound more indefinite, as if any wall were meant.

Because the whole situation is told from the past.

• Jag skulle precis mäta väggen = I was just about to measure the wall
• när Sara ringde = when Sara called

Swedish commonly uses the simple past (preterite) for both actions in this kind of past narrative.

The call from Sara happened at that past moment and interrupted or coincided with the speaker’s planned action.

Usually, no.

Jag skulle precis mäta väggen när Sara ringde normally means the speaker was just about to start measuring, but Sara called before the measuring really began.

So the sentence suggests an interrupted intention or a very near future action, not an action already in progress.

If you wanted to say the speaker was already in the middle of measuring, Swedish would more naturally use a different structure.

That version means something slightly different.

• Jag skulle precis mäta väggen = I was just about to measure the wall
• Jag höll precis på att mäta väggen = I was just measuring / I was in the middle of measuring the wall

So:

• skulle precis mäta focuses on the moment before the action started
• höll på att mäta focuses on the action already being in progress

This is an important difference.

Yes, very often you can.

• Jag skulle just mäta väggen när Sara ringde
• Jag skulle precis mäta väggen när Sara ringde

Both are natural and both can mean I was just about to measure the wall when Sara called.

In many contexts, just and precis overlap a lot.
Still, precis often feels a bit more like exactly/right then, while just can feel a bit more neutral. In this sentence, either works well.

När means when.

It introduces the time clause:

• när Sara ringde = when Sara called

This clause tells us what happened at the same time as, or at the moment that interrupted, the planned action.

So the structure is:

• main event/background: Jag skulle precis mäta väggen
• time clause/interruption: när Sara ringde

This is a very common Swedish pattern.

Not in normal English.

Even though skulle often corresponds to would in some contexts, here the best English meaning is:

• I was just about to measure the wall when Sara called
• I was just going to measure the wall when Sara called

So this is one of those cases where you should not translate skulle word-for-word as would.

Yes. Swedish main clauses usually follow the verb-second rule, meaning the finite verb comes early in the sentence.

Here the order is:

• Jag = subject
• skulle = finite verb
• precis = adverb
• mäta = infinitive
• väggen = object
• när Sara ringde = time clause

So Jag skulle precis mäta väggen is the natural order.

For example, Jag precis skulle mäta väggen would sound wrong in standard Swedish.

It shows a very common pattern used for an action that was planned or about to happen, followed by an interruption:

subject + skulle precis + infinitive + object + när + past event

For example:

• Jag skulle precis gå hem när det började regna.
• Vi skulle precis äta när gästerna kom.
• Han skulle precis somna när telefonen ringde.

So your sentence is a useful model for expressing I was just about to do X when Y happened in Swedish.