Jo, de ligger i kjølebagen.

Here jo is a reply word. It is usually used when you are:

• contradicting a negative statement
• answering a negative question
• correcting someone’s assumption

So Jo, de ligger i kjølebagen often has the feel of:

• Yes, they are
• Actually, yes
• On the contrary, they’re in the cooler bag

For example:

• De er ikke her. = They aren’t here.
• Jo, de ligger i kjølebagen. = Yes, they are — they’re in the cooler bag.

Because jo and ja are not used in exactly the same way.

• ja = ordinary yes
• jo = yes, but specifically against a negative idea

This is a very common thing for English speakers to notice, because English often just uses yes in both situations.

Compare:

• Er de i kjølebagen? = Are they in the cooler bag?
  Answer: Ja.
• Er de ikke i kjølebagen? = Aren’t they in the cooler bag?
  Answer: Jo.

So jo is the natural choice if the speaker is pushing back against a negative assumption.

Norwegian often uses position verbs where English would simply use is/are.

Here, ligger is the present tense of å ligge, meaning to lie. In Norwegian, objects that are resting somewhere, especially inside a container, are very often said to lie somewhere.

So:

• De ligger i kjølebagen = literally They lie in the cooler bag
• natural English: They are in the cooler bag

Using ligger gives a more natural Norwegian description of where the things are.

Yes. Ligger literally means lies / is lying.

But in Norwegian, this is much more normal for everyday location than in English. English usually avoids saying things like The sandwiches are lying in the cooler bag, unless we want to emphasize position. Norwegian does this much more freely.

So with objects, food, clothes, tools, and similar things, ligge is often the most natural verb.

Yes, you could, and it would be understandable.

But there is a difference in feel:

• De er i kjølebagen = more neutral, just states location
• De ligger i kjølebagen = more idiomatic and more specific about how they are located

Norwegian often prefers ligger, står, or sitter instead of just er, depending on the object and how it is positioned.

So er is possible, but ligger is often the more natural choice here.

Here de means they.

It is the subject of the sentence:

• de = they
• ligger = are lying / are
• i kjølebagen = in the cooler bag

So the basic structure is:

• De ligger i kjølebagen
• They are in the cooler bag

Because de is the subject form, while dem is the object form.

Compare:

• De ligger i kjølebagen. = They are in the cooler bag.
• Jeg ser dem. = I see them.

In your sentence, de is the thing doing the verb ligger, so the subject form is needed.

It is a compound noun:

• kjøle = cooling / to cool
• bag = bag
• kjølebag = cooler bag / insulated bag
• kjølebagen = the cooler bag

So the -en ending is the definite article built onto the noun.

Because Norwegian often puts the definite article at the end of the noun instead of using a separate word before it.

So:

• en kjølebag = a cooler bag
• kjølebagen = the cooler bag

That -en ending is doing the job that the does in English.

I means in.

It is used because the things are understood to be inside the cooler bag, not on top of it or next to it.

So:

• i kjølebagen = in the cooler bag

If the meaning were different, Norwegian would use a different preposition, such as:

• på kjølebagen = on the cooler bag
• ved kjølebagen = by the cooler bag

Because jo is functioning as a separate reply word before the main sentence.

So the structure is:

• Jo, = Yes / actually yes
• de ligger i kjølebagen = the full clause

The comma shows a small pause, just like in English when you write:

• Yes, they’re in the cooler bag.

Because jo here is not really part of the main clause grammar. It is a discourse particle or reply word placed before the clause.

So the actual clause is still:

• de ligger i kjølebagen

That is why the normal subject-verb order stays the same.

If jo were acting inside the clause in a different way, word order could become a different topic, but in this sentence it is simply a separate response word at the front.

It is singular definite:

• kjølebag = cooler bag
• kjølebagen = the cooler bag

If it were plural, it would be something different, such as:

• kjølebager = cooler bags
• kjølebagene = the cooler bags

So the sentence says the things are in one specific cooler bag.