Vi lægger revet ost på den varme pasta, så den smelter.

We put grated cheese on the hot pasta so it melts.

Breakdown of Vi lægger revet ost på den varme pasta, så den smelter.

• på: on
• vi: we
• den: it
• så: so
• den: the
• lægge: to put
• varm: hot
• pastaen: the pasta
• revet: grated
• osten: the cheese
• smelte: to melt

Questions & Answers about Vi lægger revet ost på den varme pasta, så den smelter.

Why is lægger used here, and how is it different from putter?

Lægger is the present tense of at lægge (to lay/place), often used when you place something down (especially onto a surface): lægge ost på pasta.
Putter (from at putte) is more like to put in a general sense and can sound a bit more casual: Vi putter revet ost på pastaen is possible, but lægger fits nicely with placing something on top.

Is this sentence in the present tense, and how would I say it in the past?

Yes. Vi lægger ... is present tense (we put/are putting).
Past tense is Vi lagde revet ost på den varme pasta, så den smeltede.

• lægger → lagde
• smelter → smeltede

What exactly is revet ost grammatically?

Revet is the past participle of at rive (to grate) used like an adjective, so revet ost literally means grated cheese.
It behaves like an adjective phrase modifying ost.

Why doesn’t revet have an ending like -e here?

Because revet ost is indefinite singular (no den/det/de and no definite ending on the noun). In that form, many adjectives (including participles used adjectivally) appear without -e:

• revet ost (indefinite)
  But with a definite form, you would use -e:
• den revne/ revet? (Careful:) With rive, the adjective-like forms are typically revet for the result (grated), and you’d usually keep revet: den revne is from at revne (to crack), a different verb.
  So you’d say: den revne ost is wrong; den revet ost is the right idea (though in practice you’d more often say den revne ost never; you’d say den revne doesn’t apply here). Most natural: den revne ost isn’t used; you’d rather keep it indefinite or rephrase: den revne ost is not correct; use den revne only for cracks. For cheese: den revne ost is not idiomatic—use revet ost or den revne ost not at all.

Why does it say den varme pasta and not just varm pasta?

Den varme pasta is definite: literally the hot pasta (or that hot pasta depending on context).
When an adjective comes before a definite noun in Danish, you typically use:
den/det/de + adjective(-e) + noun
So: den varm-e pasta (adjective gets -e).

Is den in den varme pasta a pronoun or an article?

Here, den functions as the definite determiner (often compared to the), used because the adjective comes before the noun: den varme pasta.
Without an adjective, Danish usually marks definiteness by adding -en/-et to the noun instead:

• pastaen = the pasta
  So you could also say: Vi lægger revet ost på pastaen ... (very common).

In så den smelter, what does den refer to?

Den refers to ost (cheese), because ost is common gender (en ost), and the pronoun for that is den.
Meaning: we put grated cheese on the hot pasta so that it melts (the cheese melts).

Could den refer to pasta instead?

Grammatically, pasta is also common gender in Danish (typically en pasta / pastaen), so den could refer to pasta, but the meaning makes it clear it’s the cheese that melts. If you wanted to avoid any ambiguity, you could say:

• ... så osten smelter. = ... so the cheese melts.

What does så mean here—so, then, or because?

Here så means so that / so in the sense of result/purpose:
..., så den smelter = ..., so that it melts / ..., so it melts.
It’s not because (that would be fordi):

• ... fordi den smelter = ... because it melts (different meaning)

Why is there a comma before så?

Danish normally uses a comma to separate the main clause from the following clause introduced by så (or similar conjunctions), especially when it’s clearly a separate clause:
Vi lægger ..., så den smelter.
Commas are a bigger topic in Danish, but this comma is very standard.

What is the word order in the så-clause? Why isn’t it like English so it melts?

It actually matches English quite closely here: så den smelter = so it melts.
Because den smelter is a normal clause: subject (den) + verb (smelter).
If you add something like ikke (not), you can see the subordinate-clause pattern more clearly:

• ..., så den ikke smelter. = ..., so that it doesn’t melt.
  (ikke comes before the verb in subordinate clauses.)

Why is på used, and could I use another preposition?

På is used for something placed on top of a surface: ost på pasta.
If it were inside something, you might use i (in):

• ost i saucen = cheese in the sauce
  So på is the natural choice for cheese sprinkled on pasta.

Could I say over instead of på?

Over usually means over/above in a spatial sense, not contact: over bordet (above the table).
For cheese sprinkled onto pasta (ending up on it), Danish uses på: ost på pasta(en).