Timeplanen ble endret i går.

Because Norwegian usually adds the definite article to the end of the noun.

• en timeplan = a timetable / a schedule
• timeplanen = the timetable / the schedule

So -en here means the.

Also, timeplan is a common-gender noun in Bokmål, which is why the indefinite article is en and the definite ending is -en.

This is a very common Norwegian passive structure.

• ble = past tense of å bli = to become
• endret = past participle of å endre = changed

Together, ble endret means was changed.

So:

• Timeplanen ble endret = The schedule was changed

This is similar to English passive grammar:

• was changed
• got changed in some contexts

Endret is the past participle of å endre.

Some useful forms are:

• å endre = to change
• endrer = change / changes
• endret = changed

In this sentence, endret is not functioning as the main finite verb by itself. It is part of the passive construction ble endret.

This is an important difference.

• ble endret focuses on the event of being changed
• var endret usually focuses more on the state/result of already being changed

So:

• Timeplanen ble endret i går = the change happened yesterday
• Timeplanen var endret i går = by yesterday, it was in a changed state

In most normal situations, if you want to say that someone changed it yesterday, ble endret is the natural choice.

Yes. It is a passive sentence.

The sentence does not say who changed the schedule. It only says that the schedule was changed.

Compare:

• Skolen endret timeplanen i går. = The school changed the schedule yesterday.
  This is active.

• Timeplanen ble endret i går. = The schedule was changed yesterday.
  This is passive.

Passive is very common in Norwegian when the person or thing doing the action is unknown, obvious, or unimportant.

Yes, sometimes, but ble endret is the most natural choice here if you mean a completed event in the past.

Norwegian also has an -s passive, for example:

• Timeplanen endres = The schedule is being changed / gets changed

But for a specific completed action in the past, ble endret is usually clearer and more common:

• Timeplanen ble endret i går.

So for this sentence, the form you have is the best one to learn first.

I går means yesterday.

In a simple main clause, Norwegian often puts time expressions toward the end:

• Timeplanen ble endret i går.

That is very natural.

You can move it for emphasis, but then Norwegian word order changes:

• I går ble timeplanen endret.

This also means The schedule was changed yesterday, but now yesterday is being emphasized.

Norwegian main clauses usually follow the V2 rule, meaning the finite verb goes in the second position.

Here:

1. Timeplanen = first element
2. ble = finite verb in second position
3. endret i går = the rest

So the structure is:

• Timeplanen | ble | endret i går

If you move i går to the front, the verb still stays second:

• I går | ble | timeplanen endret

That is a very important Norwegian pattern.

Yes. Depending on context, timeplan can often be translated as:

• timetable
• schedule
• sometimes lesson plan or class schedule

In this sentence, the schedule is probably the most natural general translation, but the timetable may be better in a school context.

Yes, you often can.

• endret = changed
• forandret = changed / altered

Both are understandable, but endret is very common and neutral in sentences like this:

• Timeplanen ble endret i går.

Using forandret would still be grammatical, but it may sound slightly less basic or slightly more stylistic depending on context.

A rough guide for an English speaker is:

• timeplanen ≈ TY-meh-plah-nen
• ble ≈ bleh
• endret ≈ EN-dret
• i går ≈ ee GOR

A few helpful notes:

• r in Norwegian is usually clearer than in most English accents
• å does not appear here, but in går the vowel is the å sound, which is a long open aw/o sound
• i går is pronounced smoothly together in normal speech

Exact pronunciation depends a bit on dialect, but this will get you close in standard East Norwegian / Bokmål-style pronunciation.