Jy maak die deeg sag deur dit lank te knie.

Jy means you when speaking to one person in an informal or neutral way. It is the normal singular subject pronoun in everyday Afrikaans.

So:

• Jy maak ... = You make ...

If you were addressing more than one person, you would usually use julle.

This is a very common Afrikaans pattern:

• maak = make
• object
• adjective

So Jy maak die deeg sag literally means You make the dough soft.

It works like English in sentences such as:

• She paints the door red
• They make it easier

In Afrikaans, sag is the adjective describing the result of what happens to die deeg.

Die is the definite article in Afrikaans, meaning the.

So:

• die deeg = the dough

A useful thing to remember is that Afrikaans uses die for:

• singular nouns
• plural nouns

There is no separate word like English the vs something else depending on number. It is simply die.

Here sag is used as a complement after maak, not directly before the noun.

Compare:

• die sagte deeg = the soft dough
  
  • here the adjective comes before the noun, so sagte
• Jy maak die deeg sag = You make the dough soft
  
  • here the adjective comes after the noun as a result-description, so just sag

This is very similar to English:

• the soft dough
• make the dough soft

Here deur means by, in the sense of by means of or through doing something.

So:

• deur dit lank te knie = by kneading it for a long time

This is a common Afrikaans structure:

• deur + object/phrase + te + infinitive

It explains how something is done.

Dit means it. It refers back to die deeg.

So:

• die deeg = the dough
• dit = it

Afrikaans often uses a pronoun like this just as English does:

• You make the dough soft by kneading it for a long time

Instead of repeating die deeg, the sentence uses dit.

This is an important difference.

• lang is usually an adjective: long
• lank is usually an adverb: for a long time / long

In this sentence, it describes how long the kneading happens, so you need the adverb:

• dit lank te knie = to knead it for a long time

Compare:

• 'n lang mes = a long knife
• Ek wag lank = I wait a long time

Te is often used before an infinitive in Afrikaans, somewhat like English to.

After deur, Afrikaans normally uses the pattern:

• deur ... te + verb

So:

• deur dit lank te knie = by kneading it for a long time

You can think of te knie as the infinitive part of the construction.

Afrikaans often puts the infinitive verb at the end in clauses like this.

The pattern here is:

• main clause: Jy maak die deeg sag
• extra phrase showing means: deur dit lank te knie

Inside that second part, the infinitive knie comes at the end. This is very normal Afrikaans word order.

So the sentence is structured like:

• You make the dough soft
• by it for a long time to knead

That last part sounds strange in English, but it is natural in Afrikaans.

Yes. In this sentence, knie is the verb to knead.

A learner may notice that knie is also the Afrikaans noun for knee. So the same written form can mean different things depending on context.

Here it is clearly a verb because of te knie and because it fits the meaning of working dough.

Not in the same way.

Om ... te is often used for to do something, especially after certain verbs or expressions:

• Ek probeer om te leer = I try to learn

But here the meaning is by doing something, and for that Afrikaans uses deur ... te:

• deur dit lank te knie = by kneading it for a long time

So deur is the right word because it explains the method.

Yes. Very much so. The sentence gives you a useful pattern:

• Jy maak [object] [adjective] deur [object/pronoun] [adverb] te [verb].

Examples:

• Jy maak die vleis sag deur dit stadig te kook.
  You make the meat tender by cooking it slowly.

• Jy maak die kamer warm deur die vensters toe te maak.
  You make the room warm by closing the windows.

• Jy maak jou Afrikaans beter deur elke dag te oefen.
  You improve your Afrikaans by practising every day.

So this sentence is good not only for vocabulary, but also for learning a very reusable grammar pattern.