Når der ikke er mere tandpasta, køber jeg en ny sammen med sæbe og shampoo.

Når means when here.

In this sentence, Når der ikke er mere tandpasta... means something like:

• When there is no more toothpaste...
• or Whenever we run out of toothpaste...

A learner might wonder why this is not Hvis (if). The difference is:

• Når is used for something expected, repeated, or seen as a normal situation.
• Hvis is more like if, for a possible condition.

So this sentence suggests a regular pattern:

• When there’s no more toothpaste, I buy a new one.

Not:

• If there happens to be no toothpaste...

In Danish, der er is the normal way to say there is / there are.

So:

• der er tandpasta = there is toothpaste
• der ikke er mere tandpasta = there is no more toothpaste

Here, der is not the same as English there in a location sense. It is part of an existential construction: it introduces the existence of something.

You would not normally say det ikke er mere tandpasta here, because det means it/that, not existential there.

Here mere means more / any more.

So:

• der er mere tandpasta = there is more toothpaste
• der ikke er mere tandpasta = there is no more toothpaste

This is a very common Danish pattern:

• Der er ikke mere kaffe. = There is no more coffee.
• Jeg har ikke mere tid. = I don’t have any more time.

A useful thing to notice is that English often uses any more in negative sentences, while Danish still uses mere.

This is because Danish is a V2 language in main clauses. That means the finite verb usually comes in the second position.

The sentence begins with a subordinate clause:

• Når der ikke er mere tandpasta

After that, the main clause starts, and the verb comes first:

• køber jeg en ny

So the structure is:

• [Subordinate clause], [verb] [subject] ...

Compare:

• Jeg køber en ny. = normal main clause
• Når der ikke er mere tandpasta, køber jeg en ny. = the first element is the whole Når... clause, so the verb køber must come before jeg

This is very typical Danish word order.

En ny means a new one.

The noun tandpasta is not repeated because Danish often leaves it out when it is obvious from context.

So:

• køber jeg en ny literally = I buy a new one
• implied meaning = I buy a new toothpaste / a new tube of toothpaste

This is very natural in Danish, just as in English:

• I need a new one.
• She bought a red one.

The noun is understood from what came before.

Because tandpasta is a common-gender noun in Danish:

• en tandpasta

When an adjective or pronoun refers back to a common-gender noun, it matches that gender:

• en ny for common gender
• et nyt for neuter gender

So:

• en tandpasta → en ny
• if it were a neuter noun, you would get et nyt

The hidden noun behind en ny is understood as something like en tandpasta or en tube tandpasta, so en is the correct form.

Because repeating it would sound unnecessary. Danish often avoids repetition when the meaning is clear.

So these are both understandable:

• køber jeg en ny
• køber jeg en ny tandpasta

But en ny is more natural if everyone already knows what is being talked about.

English does the same thing:

• When there’s no more toothpaste, I buy a new one.

Sammen med means together with or simply with.

So:

• en ny sammen med sæbe og shampoo means
  a new one together with soap and shampoo

The idea is that the speaker buys the new toothpaste at the same time as the soap and shampoo.

It does not mean that the toothpaste is physically mixed with them. It means they are bought as part of the same shopping trip or purchase.

Because sæbe and shampoo are being used in a general, uncountable way here.

So:

• sæbe = soap
• shampoo = shampoo

This is similar to English, where you can say:

• I need soap and shampoo.

You do not have to say:

• a soap
• a shampoo

unless you mean specific countable items, such as a particular bar, bottle, or type.

So in the Danish sentence, the speaker is talking generally about buying those household items.

Yes. Køber is present tense, but in Danish, present tense is often used for:

• habits
• general truths
• scheduled or expected future actions

Here it describes a habitual action:

• When we run out of toothpaste, I buy a new one.

So even though the form is present tense, the meaning is not limited to right now. It expresses what the speaker normally does in that situation.

Yes. That is a very natural way to understand it.

The phrase en ny can mean:

• a new one
• another one
• a replacement

It does not necessarily emphasize brand new in a dramatic sense. In context, it usually just means the next one you buy because the old one is used up.

So the sentence is essentially about replacing the toothpaste when it runs out.