Most mindent értek.

Because mindent is the accusative form of minden.

• minden = every / all
• mindent = everything (as a direct object)

In Most mindent értek, the verb értek means I understand, and everything is the thing being understood, so it must be in the direct-object form: mindent.

Compare:

• Minden ember fáradt. = Every person is tired.
• Mindent látok. = I see everything.

In Hungarian, the verb ending usually already shows the subject, so the pronoun is often left out.

• értek = I understand
• the ending -k shows I

So Most mindent értek already includes I without needing én.

You can say Én most mindent értek if you want emphasis, but normally én is unnecessary.

The form értek comes from the verb érteni = to understand.

Here is the basic pattern in the present tense:

• értek = I understand
• értesz = you understand
• ért = he/she understands
• értünk = we understand
• értetek = you (plural) understand
• értenek = they understand

So in this sentence, értek specifically means I understand.

This is a very common learner question, because Hungarian has indefinite and definite verb conjugation.

Even though mindent feels very specific in English, Hungarian uses the indefinite conjugation here:

• (én) értek = I understand
• not értem in this sentence

A useful thing to remember is that words like mindent often go with the indefinite form in Hungarian:

• Mindent tudok. = I know everything.
• Mindent látok. = I see everything.
• Mindent értek. = I understand everything.

So this is a normal and correct pattern.

No, Hungarian most does not mean the same thing as English most.

Hungarian most means now.

So:

• most = now

This is just a coincidence in spelling. It is a false friend for English speakers.

Yes, Hungarian word order is flexible, but changing it changes the focus or emphasis.

• Most mindent értek. = Now I understand everything.
  Neutral, with most setting the time frame.

• Mindent értek most. = I understand everything now.
  Also possible, with a slightly different rhythm or emphasis.

• Most értek mindent. = It is now that I understand everything.
  This can sound more focused or contrastive.

So the original sentence is very natural, but other orders are possible depending on what the speaker wants to highlight.

Putting most first gives the sentence a natural time-setting feel: As for now...

Hungarian often places time expressions early in the sentence, especially when they set the scene.

So Most mindent értek feels like:

• Now, I understand everything
• At this point, I understand everything

It is a very natural order.

A rough pronunciation guide:

• most ≈ mosht
• mindent ≈ MIN-dent
• értek ≈ AIR-tek

A few details:

• s in Hungarian is pronounced like English sh
• sz is the English s sound
• é is a long vowel, somewhat like ay in say, but steadier

So most is not pronounced like English most; it sounds more like mosht.

In meaning, it is usually just everything.

Hungarian often uses minden-words in ways that map neatly onto English:

• minden = every / all
• mindent = everything
• mindenki = everyone
• mindenhol = everywhere

So in this sentence, the most natural English equivalent is simply everything.

Yes, but it means something slightly different.

• Most mindent értek. = Now I understand everything.
• Már mindent értek. = I understand everything already / now at last.

So:

• most focuses on the present moment
• már adds the idea of already or by now

Both are common, but they are not identical.

Yes, and it is very natural.

• Most már mindent értek. = Now I understand everything / Now I finally understand everything.

Here:

• most = now
• már = already / by now / at last

Together they often give the feeling of a new understanding that has just been reached.

For many situations, Most már mindent értek sounds even more idiomatic than the shorter sentence, depending on context.