Endurvinnslan hefur góð áhrif á umhverfið.

-n is the suffixed definite article for many feminine nouns in Icelandic.

• endurvinnsla = recycling (indefinite)
• endurvinnslan = the recycling (definite)

Using the definite form here often makes the statement sound like a general, established concept: the practice/system of recycling.

It’s a noun meaning recycling, built from:

• endur- = re- / again
• vinnsla = processing, working, production (a noun from the verb vinna = to work/do)

So endurvinnsla is literally something like re-processing.

Because the subject Endurvinnslan is singular (3rd person singular).
The verb hafa (to have) conjugates as:

• ég hef (I have)
• þú hefur (you have)
• hann/hún/það hefur (he/she/it has)

So: Endurvinnslan hefur … = Recycling has …

Yes—hefur is present tense. Icelandic often uses present tense for general truths and habitual facts, just like English:

• Endurvinnslan hefur … = Recycling has / Recycling does have (in general)

Yes, hafa áhrif á + accusative is the standard, very common pattern meaning to have an effect/influence on:

• hafa áhrif á eitthvað = to affect something / influence something

It’s one of those fixed combinations you’ll see a lot.

Áhrif is very commonly used in the plural to mean effects / influence, even when English might use singular effect. A singular áhrif exists but is much less common in everyday usage.
So góð áhrif is the normal way to say a positive effect / good effects.

Because áhrif is neuter plural, and the adjective must agree in gender + number + case.

Here it’s (effectively) accusative plural after hafa, and for neuter plural the adjective form is góð:

• masculine plural: góðir
• feminine plural: góðar
• neuter plural: góð

So: góð áhrif is the correct agreement.

umhverfið is umhverfi (environment) with the definite article:

• umhverfi = environment (indefinite)
• umhverfið = the environment (definite)

With hafa áhrif á, the noun after á is normally accusative, and here it’s also definite: á umhverfið.

Correct: á can govern either accusative or dative, depending on meaning. But in the fixed expression hafa áhrif á, it takes accusative:

• hafa áhrif á + accusative = to affect / have an influence on

So umhverfið is in the accusative form (which happens to look the same as nominative for this word in the definite neuter singular).

Yes. Both can be natural, with a slight nuance:

• Endurvinnsla hefur … = Recycling (as a general activity) has …
• Endurvinnslan hefur … = The recycling (as a system/practice being discussed) has …

In many contexts, both will be understood the same way.

Both mean positive effects, but:

• góð áhrif = good effects (more general, very common)
• jákvæð áhrif = explicitly positive effects (a bit more formal/precise, common in writing)

A rough guide (regional variation exists):

• Endurvinnslan: EN-dur-vin-slan (the nn is pronounced like a long n)
• hefur: HEH-vur (often with a relaxed vowel)
• góð: gohth (with ð like the th in this, but often softer)
• áhrif: OW-riv (the hr is voiceless; many learners approximate it as r)
• umhverfið: UM-kveh-rith (with hv like kv, and final ð soft)