Hon använder linjalen när hon ritar ett hus, men hon använder suddgummit när det blir fel.

Because Swedish usually marks definiteness by adding an ending to the noun.

• linjal = ruler

• linjalen = the ruler

• suddgummi = eraser
• suddgummit = the eraser

The ending depends on the noun’s gender:

• en-word nouns often take -en in the definite singular
• ett-word nouns often take -et in the definite singular

So here:

• linjal is an en word → linjalen
• suddgummi is an ett word → suddgummit

Swedish does also use separate words like den/det, but mainly when there is an adjective, for example:

• den långa linjalen = the long ruler

Because ett hus means a house, not the house.

In this sentence, she is drawing a house in general, not one specific house that has already been identified. So Swedish uses the indefinite form:

• ett hus = a house
• huset = the house

If you said hon ritar huset, that would mean she is drawing the house, probably a specific one already known from the context.

Because Swedish needs a subject in that clause too.

The sentence has two main parts:

• Hon använder linjalen när hon ritar ett hus
• men hon använder suddgummit när det blir fel

In när hon ritar ett hus, the clause hon ritar ett hus needs its own subject, so hon must be there.

This is actually similar to English:

• She uses the ruler when she draws a house

You also repeat she in English.

It can feel like both.

In sentences about habits or repeated situations, Swedish när often corresponds to English when or whenever:

• när hon ritar ett hus = when / whenever she draws a house
• när det blir fel = when / whenever it goes wrong

So even though English sometimes prefers whenever, Swedish usually just uses när.

Because när presents it as something that happens in that situation, while om would sound more conditional: if it happens.

• när det blir fel = when it goes wrong / when it turns out wrong
• om det blir fel = if it goes wrong

In this sentence, när fits well because it describes what she does in that kind of situation: she uses the eraser when mistakes happen.

Here det is an impersonal subject. It does not necessarily refer to one specific thing. It is similar to English expressions like:

• it goes wrong
• it gets bad
• it happens

So:

• det blir fel literally looks like it becomes wrong
• natural English: it goes wrong, there is a mistake, or something turns out wrong

This is a very common Swedish expression.

Bli fel is a common Swedish expression meaning:

• to go wrong
• to turn out wrong
• to be incorrect by mistake

So:

• det blir fel = something goes wrong / it comes out wrong

It is very idiomatic. Swedish speakers use it a lot for mistakes in drawing, writing, calculations, printing, cooking, and so on.

Examples:

• Det blev fel i texten. = There was a mistake in the text.
• Nu blev det fel igen. = Now it went wrong again.

Because Swedish uses the present tense for general habits and repeated actions, just like English often does.

This sentence describes what she generally does:

• she uses the ruler when she draws a house
• she uses the eraser when something goes wrong

So the present tense is the natural choice.

A past version would be:

• Hon använde linjalen när hon ritade ett hus, men hon använde suddgummit när det blev fel.

Because men is a coordinating conjunction, and it does not force inversion.

So the normal order stays:

• men hon använder suddgummit

However, Swedish does use inversion when something else is placed first in a main clause. For example:

• När hon ritar ett hus, använder hon linjalen.

Here the time clause När hon ritar ett hus comes first, so the main clause becomes:

• använder hon not
• hon använder

That is a very important Swedish word-order pattern.

Because rita ett hus means to draw a house as the object of the drawing.

• rita ett hus = draw a house
• rita en bil = draw a car
• rita ett ansikte = draw a face

But rita på means draw on a surface:

• rita på väggen = draw on the wall
• rita på papperet = draw on the paper

So the sentence uses rita ett hus because the house is what she is drawing.

You mostly have to learn the gender together with each noun.

In this sentence:

• en linjal = a ruler
• ett suddgummi = an eraser
• ett hus = a house

That gives:

• linjalen = the ruler
• suddgummit = the eraser
• huset = the house

There are some patterns in Swedish noun gender, but there are many exceptions, so the safest habit is to memorize nouns with their article:

• learn en linjal, not just linjal
• learn ett hus, not just hus

It is a correct and common word, but in everyday speech many speakers also say sudd.

So:

• ett suddgummi = an eraser
• suddet / suddgummit = the eraser

Both are fine, but sudd can sound more casual and everyday, especially in speech.

Not very naturally in this exact sentence.

Swedish likes to repeat the verb here because the sentence contrasts two actions clearly:

• Hon använder linjalen ... men hon använder suddgummit ...

That repetition makes the structure balanced and easy to follow.

You might sometimes omit repeated material in other contexts, but here the full repetition sounds natural and clear.