Löken blev hackad så fint att barnen inte märkte den i maten.

Löken is the definite singular form of lök and means the onion.

• lök = onion
• en lök = an onion
• löken = the onion

Swedish usually adds the definite article as an ending, so instead of a separate word like the, you often get a suffix such as -en or -et.

Blev hackad means was chopped or more literally became chopped.

This is a very common Swedish way to express a passive meaning:

• bli = to become
• blev = became
• hackad = chopped

So löken blev hackad means the onion underwent the action of being chopped.

Because hackad agrees with löken, which is an en-word and singular.

Swedish participles often behave like adjectives and agree with the noun:

• en lök → hackad
• ett äpple → hackat
• lökarna → hackade

Since löken is singular and common gender, hackad is the correct form.

Swedish has more than one way to express the passive.

Two common options are:

• Löken blev hackad
• Löken hackades

Both can mean the onion was chopped.

The bli + participle form often feels a bit more dynamic and focuses on the result of something happening. The -s passive is also common, but in everyday speech blev hackad is very natural.

Så ... att means so ... that.

So:

• så fint = so finely / so nicely
• att barnen inte märkte den i maten = that the children did not notice it in the food

This construction is used just like English so ... that:

• Han var så trött att han somnade. = He was so tired that he fell asleep.
• Löken blev hackad så fint att ... = The onion was chopped so finely that ...

Here fint works like an adverb and means finely.

The basic adjective is:

• fin = fine, nice

But Swedish often uses the -t form to express how something is done:

• sjunga fint = sing beautifully
• skära fint = cut neatly
• hacka fint = chop finely

So så fint describes the manner of chopping, not the onion itself.

Because after att, Swedish uses subordinate clause word order.

In a subordinate clause, the subject usually comes before inte:

• att barnen inte märkte den = that the children did not notice it

Compare that with a main clause:

• Barnen märkte inte den.

So this sentence is a good example of the normal Swedish rule:

• main clause: verb before inte
• subordinate clause: inte before the verb

Den refers back to löken.

Since lök is an en-word, the pronoun is den:

• en lök → den
• ett äpple → det

So barnen inte märkte den means the children did not notice it, where it = the onion.

Yes. Märkte is the past tense of märka, and here it means noticed.

• märka = notice
• märker = notice / notices
• märkte = noticed

This verb can confuse English speakers because it may look a bit like mark, but in this context it definitely means notice.

Barnen means the children.

• barn can mean children in an indefinite sense
• barnen means the children, specific children

So the sentence is talking about a particular group of children, not children in general.

I maten means in the food or in the dish/meal.

Swedish often uses the definite form when referring to a specific meal or food being eaten in that situation. So maten here is not food in general, but the actual food on the table.

Compare:

• mat = food in general
• maten = the food / the meal

Yes.

• blev = past tense of bli
• märkte = past tense of märka

So the whole sentence describes a completed event in the past: the onion was chopped very finely, and the children did not notice it.

Yes. Swedish can add the person doing the action with av.

For example:

• Löken blev hackad av mamman. = The onion was chopped by the mother.

But in your sentence, the person who did the chopping is not important, so Swedish leaves that out. The focus is on the result: the onion was chopped so finely that the children did not notice it.