A kopogás a zajforrás.

In Hungarian, the verb to be is often omitted in the present tense, third person when you are linking two nouns or a noun and an adjective.

So Hungarian says:

A kopogás a zajforrás.

where English would say:

The knocking is the noise source.

This is completely normal Hungarian grammar.

If the sentence were in the past, or in first or second person, you would see a form of van more clearly. But in a simple present-tense statement like this, Hungarian usually leaves it out.

Because a is the singular definite article, meaning the.

So:

• a kopogás = the knocking
• a zajforrás = the noise source

Hungarian often uses the definite article with both parts of this kind of sentence when both are definite. English can do something similar, but Hungarian does it very naturally here.

Hungarian has two forms of the definite article:

• a before a consonant sound
• az before a vowel sound

Here both nouns begin with consonants:

• kopogás starts with k
• zajforrás starts with z

So the correct article is a in both cases.

Examples:

• a ház = the house
• az autó = the car

Kopogás is a noun. It comes from the verb kopog, which means to knock or to make a knocking sound.

The ending -ás / -és often turns a verb into an action noun, similar to English -ing in some cases.

So:

• kopog = knock
• kopogás = knocking, a knocking sound

In this sentence, kopogás refers to the sound or act of knocking, not to a person doing the knocking.

Because it is a compound noun, and Hungarian very often writes compounds as a single word.

It is made of:

• zaj = noise
• forrás = source

So:

zajforrás = noise source

This is very common in Hungarian. English often uses two words or an of phrase where Hungarian prefers a compound.

So instead of something like source of noise, Hungarian can simply say zajforrás.

Yes, that is also grammatical, but the information focus changes.

Hungarian word order is strongly connected to what is already known and what is being identified or emphasized.

• A kopogás a zajforrás.
  = As for the knocking, it is the noise source.

• A zajforrás a kopogás.
  = As for the noise source, it is the knocking.

The basic meaning is very similar, but the starting point of the sentence changes.

A rough English-friendly pronunciation would be:

aw KO-po-gaash aw zoy-for-raash

A few helpful points:

• Stress in Hungarian usually falls on the first syllable of each word.
• á is a long vowel, somewhat like the a in father.
• s in Hungarian sounds like English sh.
• j in zaj sounds like English y.

So:

• kopogás sounds roughly like KO-po-gaash
• zajforrás sounds roughly like zoy-FOR-raash

They are closely related, but not exactly the same in form.

• zajforrás is a compound noun meaning noise source
• a zaj forrása literally means the source of the noise

So in many contexts they can be very similar, but grammatically they are built differently.

In your sentence, a zajforrás is a compact, natural Hungarian way to express the idea.

Yes. This kind of sentence, where one noun is identified as another noun, is very common.

Hungarian often uses this pattern:

A X a Y.

with no visible is in the present tense.

For example:

• A kutya az állat. = The dog is an animal.
• Péter a tanár. = Péter is the teacher.
• A kopogás a zajforrás. = The knocking is the noise source.

So this sentence is a good example of a very important Hungarian structure.