Jag lägger en tunn skiva ost på smörgåsen, eftersom hon inte vill ha skinka i dag.

Lägger is the present tense of lägga, which often means to put, to lay, or to place.

In this sentence, it is the natural verb for putting a slice of food onto bread. For toppings on a sandwich, lägga is much more natural than sätta.

A few useful forms:

• lägga = infinitive
• lägger = present tense
• lade = past tense
• lagt = supine/past participle-related form

So Jag lägger ... means I am putting / I put ...

Because skiva is an en-word in Swedish.

In the singular indefinite form:

• with an en-word, the adjective usually has its basic form: en tunn skiva
• with an ett-word, the adjective usually adds -t: ett tunt bröd

Compare:

• en tunn skiva
• ett tunt lager
• den tunna skivan
• tunna skivor

So tunn agrees correctly with en skiva.

This is a very common Swedish pattern. When Swedish talks about a portion, measure, or piece of something, it often uses noun + material/contents without a preposition.

Examples:

• en skiva ost
• en kopp kaffe
• en bit bröd
• ett glas vatten

So en skiva ost is the normal way to say a slice of cheese.

Using av is possible in some contexts, but it usually sounds more literal or emphasizes what something is made of, rather than just naming a food portion.

Because smörgåsen is the definite form: the sandwich.

Swedish usually adds the definite article to the end of the noun:

• en smörgås = a sandwich
• smörgåsen = the sandwich

Here, the speaker is talking about a specific sandwich, probably the one they are making right now, so the definite form is natural.

If it were not specific, you might say:

• på en smörgås = on a sandwich

Because eftersom introduces a subordinate clause, and subordinate clauses have different word order from main clauses in Swedish.

In a main clause, you would usually say:

• Hon vill inte ha skinka i dag.

But after eftersom, Swedish uses subordinate clause word order:

• eftersom hon inte vill ha skinka i dag

A useful rule:

• main clause: subject + verb + inte
• subordinate clause: subject + inte
  • verb

That is why inte comes before vill here.

Because vill ha is the normal Swedish way to say that someone wants something.

Literally, ha means have, but together:

• vill ha = want

Examples:

• Jag vill ha kaffe.
• Hon vill ha ost.
• Vi vill ha mer tid.

Also, after vill, Swedish normally does not use att:

• hon vill ha
• not hon vill att ha

So hon inte vill ha skinka is the standard structure.

Because skinka here is being used as a mass noun, meaning an unspecified amount of ham in general.

That is very similar to English:

• she doesn’t want ham today
• not usually she doesn’t want a ham today

So in Swedish:

• ha skinka = have/want ham
• not necessarily ha en skinka, which would mean something more like have a ham as a whole item

If you were talking about a specific ham, then a definite form or another determiner could be used:

• den skinkan
• skinkan

But here the general, article-less form is correct.

Yes. Both are used, and both mean today.

You will see:

• i dag
• idag

The one-word form idag is very common in modern Swedish, while i dag is also correct and still widely used. The difference is mainly a spelling/style issue, not a meaning difference.

Yes, you could.

Both are natural:

• en tunn skiva ost
• en tunn ostskiva

The version in your sentence, en tunn skiva ost, uses a very common measure/portion pattern. It can sound especially natural in everyday speech.

Ostskiva is also perfectly normal, and many speakers would use it too. So this is more about style and phrasing than right versus wrong.

Then the sentence would be:

Eftersom hon inte vill ha skinka i dag, lägger jag en tunn skiva ost på smörgåsen.

Notice what happens in the main clause after the subordinate clause:

• lägger jag
• not jag lägger

That is because Swedish main clauses follow the V2 rule: the finite verb comes in the second position. If the sentence begins with Eftersom hon inte vill ha skinka i dag, that whole clause takes the first position, so the verb lägger must come before jag.

The sk in skiva and skinka is not pronounced like the English sk in skin.

Because sk comes before i, Swedish uses the special sj-sound here. This sound does not have an exact English equivalent, but many learners first approximate it with something like sh.

So:

• skiva begins with that Swedish sj-like sound
• skinka does too

Also, a couple of other pronunciation notes:

• jag is often pronounced more like ya in everyday speech
• smörgåsen is stressed on the first syllable: SMÖR-gå-sen

The exact pronunciation varies somewhat by region, but those points will help you sound more natural.