Erstatningsbussen reduserer ventetiden.

Because Norwegian very often makes compound nouns by joining words together.

• erstatning = replacement
• buss = bus

Together they form erstatningsbuss = replacement bus.

Then the ending -en is added to make it definite:

• erstatningsbuss = a replacement bus
• erstatningsbussen = the replacement bus

So erstatningsbussen is not unusual at all—it is a normal Norwegian compound noun.

That -s- is a linking sound/letter often used in Norwegian compounds.

So:

• erstatning
  • buss → erstatningsbuss

It does not mean possession here. It is just part of how the compound is formed. English learners often expect something more literal like replacement bus, but Norwegian commonly joins the words and may insert an -s- between them.

In Norwegian, the is usually attached to the end of the noun instead of standing as a separate word.

So:

• buss = bus
• bussen = the bus

And similarly:

• ventetid = waiting time
• ventetiden = the waiting time

That is why you see:

• erstatningsbussen = the replacement bus
• ventetiden = the waiting time

This is one of the biggest structural differences from English.

Ventetiden is the definite form of ventetid.

• ventetid = waiting time
• ventetiden = the waiting time

Using ventetiden suggests a specific or understood waiting time—the waiting time relevant in this situation.

If you said Erstatningsbussen reduserer ventetid, it would sound unnatural in ordinary Norwegian. The definite form fits much better here.

Reduserer is the present tense of å redusere = to reduce.

So:

• å redusere = to reduce
• reduserer = reduce / reduces / is reducing, depending on context

In this sentence, it means reduces.

Because Norwegian verbs do not change for person the way English verbs do.

In English:

• I reduce
• you reduce
• he/she reduces

In Norwegian, the present tense stays the same:

• jeg reduserer
• du reduserer
• han/hun reduserer

So reduserer works with all subjects.

No—this is the normal Norwegian subject–verb–object order.

• Erstatningsbussen = subject
• reduserer = verb
• ventetiden = object

So the structure is very similar to English:

• The replacement bus reduces the waiting time.

This sentence is straightforward and not using any unusual word order.

Yes, grammatically you could, but the meaning changes slightly.

• Erstatningsbussen = the replacement bus
• En erstatningsbuss = a replacement bus

So:

• Erstatningsbussen reduserer ventetiden = The replacement bus reduces the waiting time.
• En erstatningsbuss reduserer ventetiden = A replacement bus reduces the waiting time.

The original sentence points to a specific replacement bus, not just any one.

Yes. Ventetid is made from:

• vente = to wait
• tid = time

Together, ventetid means waiting time.

Then the definite ending gives:

• ventetid = waiting time
• ventetiden = the waiting time

So this short sentence actually contains two very typical Norwegian compounds: erstatningsbuss and ventetid.

A native speaker would likely understand it as something like:

• the replacement bus helps shorten the waiting time
• the replacement bus makes people wait less

It sounds like a practical statement, for example in a public transport context, where a replacement bus has been provided because a train or regular bus is not running.