A dioptria alacsony.

In Hungarian, the verb to be is usually left out in the present tense when the subject is third person.

So:

• A dioptria alacsony = The diopter/prescription is low
• not A dioptria van alacsony

This is very normal in Hungarian.
You do use forms of van in other situations, but not in simple present-tense sentences like this one.

Compare:

• Én magas vagyok. = I am tall.
• Te fáradt vagy. = You are tired.
• Ő magas. = He/she is tall.

Notice that in third person singular, there is no separate word for is.

A is the definite article, meaning the.

Hungarian has two forms of the definite article:

• a
• az

Which one you use depends on the sound at the start of the next word:

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

Since dioptria starts with d, a consonant sound, Hungarian uses a:

• a dioptria

If the next word began with a vowel, you would use az instead.

Because dioptria begins with a consonant sound: d.

Hungarian definite articles work by sound, not by the letter name or by grammatical gender:

• a könyv = the book
• a dioptria = the diopter/prescription
• az alma = the apple

So a dioptria is correct simply because dioptria starts with a consonant sound.

No. Hungarian adjectives do not agree with nouns the way they do in many Indo-European languages.

So alacsony stays the same regardless of gender, and Hungarian does not have grammatical gender anyway.

Examples:

• A ház alacsony. = The house is low/short.
• A lány alacsony. = The girl is short.
• A férfi alacsony. = The man is short.

The adjective alacsony itself does not change.

It is singular here.

There is no plural ending on dioptria in this sentence. Hungarian plurals are usually marked with -k, but that is not present here.

So:

• A dioptria alacsony. = singular
• a plural version would look different

Also, when talking about eyeglass prescription, Hungarian often uses dioptria in a somewhat measure-like way, so the exact phrasing can depend on context.

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

For example:

• A dioptria alacsony.
  Neutral statement: The prescription is low.
• Alacsony a dioptria.
  More focus on low: The prescription is low / It’s low, the prescription.

Both can be correct, but they are not always identical in tone.
The version with A dioptria alacsony sounds like a plain, straightforward statement.

The sentence breaks down like this:

• A = definite article, the
• dioptria = subject noun
• alacsony = predicate adjective, describing the subject

So structurally it is:

• the + noun + adjective

Even though English needs is, Hungarian does not include it here in the present tense third person.

No. Although alacsony often means short or low in physical descriptions, it can also be used more generally for something that is at a low level, degree, or amount.

So it can describe:

• physical height
• level
• amount
• numerical value

In this sentence, it describes the level/value of the diopter or prescription, not physical height.

A rough English-friendly pronunciation is:

dee-op-tree-aw

More carefully in Hungarian, it is approximately:

di-op-tri-a

A few notes:

• i is like ee
• o is a short o
• a at the end is a short Hungarian a, not exactly like English ah, but close enough for a beginner

Hungarian pronunciation is usually quite regular, so once you know the sounds, words are generally pronounced as written.

Yes. It is a complete and grammatical Hungarian sentence.

Even though it may feel incomplete to an English speaker because there is no written is, Hungarian speakers understand it as a full sentence:

• subject: A dioptria
• predicate: alacsony

So from a Hungarian grammar point of view, nothing is missing.