Мы сидели под навесом и слушали, как шумит лес.

Russian can drop subject pronouns, but here мы helps make the subject clear.

A useful detail: in the past tense, Russian verbs show number and, in the singular, gender, but not person. So сидели could mean:

• we were sitting
• you (plural) were sitting
• they were sitting

Because of that, мы is very natural here.

Both are past tense plural forms:

• сидели = were sitting / sat
• слушали = were listening / listened

They end in -ли, which is the normal plural past-tense ending.

Compare:

• я сидел / сидела = I was sitting
• мы сидели = we were sitting
• она слушала = she was listening
• они слушали = they were listening

So the form matches мы.

Because под can take different cases depending on the meaning:

• под + Instrumental = location, under something
• под + Accusative = motion to a position under something

Here the meaning is static location: they were sitting under a shelter/canopy, so Russian uses the instrumental:

• под навесом = under the canopy / under the shelter

Compare:

• сидеть под навесом = to sit under the shelter
• идти под навес = to go under the shelter

Навес is a kind of simple covered structure: a canopy, awning, lean-to, or shelter with a roof. It usually suggests protection from sun or rain, but not a fully enclosed building.

So под навесом gives the image of being under some kind of cover outdoors.

This is a very common and important distinction:

• слушать = to listen (active, intentional)
• слышать = to hear (perceive sound)

So:

• Мы слушали, как шумит лес. = We were listening to the forest.
• Мы слышали, как шумит лес. = We heard the forest making noise.

The original sentence stresses that they were paying attention to the sound, not just noticing it.

Because как шумит лес is a subordinate clause.

The main clause is:

• Мы сидели под навесом и слушали
  = We were sitting under the shelter and listening

Then comes what they were listening to:

• как шумит лес
  = how the forest sounds / how the forest is rustling

Russian normally puts a comma before this kind of subordinate clause.

Yes, как literally means how, but in this sentence it functions as a conjunction introducing what was being heard/listened to.

So слушали, как шумит лес is literally something like:

• listened to how the forest rustles

In natural English, we usually say:

• listened to the forest rustling
• listened to the sound of the forest
• listened as the forest rustled

So как is not asking a question here; it is introducing the sound/event being perceived.

Because Russian does not follow sequence of tenses the same way English does.

After verbs of perception such as слышать, слушать, видеть, Russian often uses the present tense in the subordinate clause for something happening at that same time:

• Мы слушали, как шумит лес.

This makes the sound feel immediate and alive: while we were sitting there, the forest is rustling.

In English, we usually shift this to the past:

• We listened to the forest rustling
• We listened as the forest rustled

Could Russian also say как шумел лес? Yes, that is possible too. It would sound a bit more like a regular past-time narrative. Шумит often feels more vivid.

Both are possible, because Russian word order is flexible.

• лес шумит = the forest is rustling/noisy
• шумит лес = the forest is rustling/noisy

In как шумит лес, putting the verb first can sound smoother and slightly more descriptive or literary. It also puts the listener’s attention first on the sound/action, then on its source.

Grammatically, лес is still the subject, in the nominative case.

Because the sentence describes ongoing background actions, not single completed events.

• сидели = were sitting
• слушали = were listening
• шумит = is rustling / is making noise

These are all processes happening over some stretch of time.

If you changed the aspect, the meaning would change:

• сели = sat down
• послушали = listened for a while / listened once
• зашумел = began to make noise

So the imperfective forms are the natural choice for a calm, continuous scene.