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

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?