Mathjelpen kommer fra matdonasjonen.

Because Norwegian normally writes compound nouns as a single word.

• mathjelp = mat
  • hjelp
• matdonasjon = mat
  • donasjon

The first part narrows the meaning of the second part:

• mathjelp = a kind of help
• matdonasjon = a kind of donation

This is very common in Norwegian. English often uses two words where Norwegian prefers one compound word.

The ending -en marks the definite singular form, which is often the equivalent of English the.

So:

• mathjelp = food aid
• mathjelpen = the food aid

and

• matdonasjon = food donation
• matdonasjonen = the food donation

Instead of putting a separate word before the noun, Norwegian often adds the definite ending directly to the noun.

It is built into the nouns themselves.

In Norwegian, the definite article is very often attached to the end of the noun:

• hjelpen = the help
• donasjonen = the donation

So mathjelpen already means the food aid, and matdonasjonen already means the food donation.

kommer fra literally means comes from, but here it means originates from or is provided through/from.

So the sentence is saying that the source of the food aid is the food donation.

This is a normal use of komme fra in Norwegian, both for physical origin and more abstract source.

kommer is the present tense of komme = to come.

In this sentence, the present tense is used to state a fact or current relationship:

• Mathjelpen kommer fra matdonasjonen.
• The food aid comes from the food donation.

Norwegian uses the present tense in many of the same ways English uses the simple present.

In Norwegian compounds, the last part is the main noun, and the first part describes it.

So in mathjelp:

• hjelp is the core noun
• mat tells you what kind of help it is

That means mathjelp is literally food-help, or more naturally in English, food aid.

The same logic applies to matdonasjon:

• donasjon is the core noun
• mat tells you what kind of donation it is

This is the normal word order for a basic Norwegian main clause:

• subject
  • verb
    • other information

Here:

• Mathjelpen = subject
• kommer = verb
• fra matdonasjonen = prepositional phrase telling where it comes from

So the sentence follows a very standard pattern.

Yes, but Norwegian main clauses still usually keep the verb in second position.

For example:

• Fra matdonasjonen kommer mathjelpen.

This is grammatically possible, but it is more marked and puts extra focus on fra matdonasjonen.

That verb-second pattern is an important feature of Norwegian word order.

Yes. The sentence as written refers to specific food aid and a specific food donation.

• mathjelpen = the food aid
• matdonasjonen = the food donation

If you made them indefinite, the meaning would become more general or less specific:

• Mathjelp kommer fra en matdonasjon.
• Food aid comes from a food donation.

So the definite endings matter a lot.

Yes, you could. Both can express a similar idea:

• matdonasjonen = the food donation
• donasjonen av mat = the donation of food

The compound matdonasjonen is shorter and often feels more compact and natural as a noun phrase.
The version with av can sound a little more analytical or explicit.

A rough guide:

• mathjelpen: the hj at the start of hjelp is usually pronounced close to English y in yes, so hjelp sounds roughly like yelp, though with a Norwegian vowel quality
• matdonasjonen: the -sjon part is pronounced roughly like shoon

So very roughly:

• mathjelpen ≈ maht-yel-pen
• matdonasjonen ≈ maht-do-na-shoo-nen

These are only approximations, but they are helpful for getting started.