Ni fritas fungojn.

Because it is both:

• plural: -j = more than one mushroom
• the direct object: -n = the thing being fried

So:

• fungo = a mushroom
• fungoj = mushrooms
• fungon = a mushroom (as the direct object)
• fungojn = mushrooms (as the direct object)

In Ni fritas fungojn, fungojn is what we fry, so it takes -n.

Because ni is the subject, not the object.

In this sentence:

• Ni = the ones doing the action
• fritas = fry / are frying
• fungojn = the thing receiving the action

Only the direct object gets -n, so ni stays just ni.

-as is the Esperanto present-tense verb ending.

So:

• friti = to fry
• fritas = fry / am frying / is frying / are frying

Esperanto does not change the verb for different subjects the way English does. So:

• mi fritas = I fry / I am frying
• vi fritas = you fry / you are frying
• ni fritas = we fry / we are frying

The verb form stays fritas.

It can mean either one, depending on context.

Esperanto -as covers the general English present and the progressive present:

• We fry mushrooms
• We are frying mushrooms

Usually the situation makes it clear which one is meant.

Esperanto often leaves that unspecified unless it needs to be clear.

So fungojn can mean:

• mushrooms
• some mushrooms
• the mushrooms

If you want to be specific, you can add words such as:

• la fungojn = the mushrooms
• kelkajn fungojn = some mushrooms

But without extra context, plain fungojn is perfectly normal.

Yes, Esperanto word order is fairly flexible because the -n ending shows the object.

So these can all be understood:

• Ni fritas fungojn.
• Fungojn ni fritas.

Both still mean that we are frying mushrooms, because fungojn has the object marker -n.

That said, Ni fritas fungojn is the most neutral and natural order.

Normally, no.

Ni fritas fungoj is incorrect in standard Esperanto because fungoj is the direct object and should therefore be fungojn.

The -n is very important for showing grammatical role.

The dictionary forms are:

• friti = to fry
• fungo = mushroom

Esperanto words are often built from a stable root plus endings:

• frit-
  • -i → friti
• frit-
  • -as → fritas
• fung-
  • -o → fungo
• fung-
  • -oj → fungoj
• fung-
  • -ojn → fungojn

This regularity is one of the most helpful features of Esperanto.

A simple approximation is:

FOON-goyn

More exactly:

• fu sounds like foo
• ng as in sing
• oj sounds like oy in boy
• final n is pronounced

So fungojn is roughly FOON-goyn.

Usually you should include ni.

Esperanto verbs do not show the subject by themselves, so fritas alone only tells you the tense, not who is doing it. Because of that, pronouns are normally stated unless the subject is obvious from context.

So the clear full sentence is:

• Ni fritas fungojn. = We are frying mushrooms.