גניבת זהות היא הונאה רצינית.

A natural pronunciation is:

g'nei-VAT ze-HUT hi ho-na-AH re-tsi-NIT

A rough breakdown:

• גניבת = g'nei-VAT
• זהות = ze-HUT
• היא = hi
• הונאה = ho-na-AH
• רצינית = re-tsi-NIT

The stress is mostly on the last syllable of each main word.

Because גניבת is the construct form of גניבה.

• גניבה = theft
• גניבת = theft of ...

Hebrew often changes a noun when it is followed by another noun in an of relationship. So:

• גניבה = theft
• גניבת זהות = theft of identity = identity theft

This structure is very common in Hebrew.

Hebrew usually does not use a separate word for of in this kind of phrase. Instead, it uses a structure called the construct chain.

So:

• גניבת זהות literally means theft of identity
• in natural English, that becomes identity theft

This is one of the most important differences between Hebrew and English noun phrases.

Great question. היא does usually mean she, but in sentences like this it also works as a kind of linking word between two nouns.

Hebrew often does not use a normal present-tense verb for to be. In noun-to-noun sentences, Hebrew commonly inserts הוא or היא.

So here:

• גניבת זהות = identity theft
• היא = is
• הונאה רצינית = a serious fraud

So היא is functioning like is, even though its basic form is the pronoun she.

Because the subject גניבת זהות is grammatically feminine singular.

The head noun is גניבה, which is feminine. In the construct form, it becomes גניבת, but it keeps its feminine gender.

So Hebrew uses:

• היא for feminine singular
• הוא for masculine singular

That is why the sentence has היא, not הוא.

Because it describes הונאה, and הונאה is a feminine noun.

Hebrew adjectives must agree with the noun they describe in gender and number.

So:

• masculine singular: רציני
• feminine singular: רצינית

Since הונאה is feminine singular, the adjective must be רצינית.

Grammatically, רצינית describes הונאה.

The sentence is structured like this:

• גניבת זהות = subject
• היא = linking word
• הונאה רצינית = predicate noun phrase

So the meaning is:

Identity theft is a serious fraud

Not:

identity theft is serious

Even though the overall idea is similar, the adjective is attached directly to הונאה.

Because the sentence is talking about identity theft in general, not a specific case.

So:

• גניבת זהות = identity theft, in a general sense
• הונאה רצינית = a serious fraud

Hebrew often leaves nouns without ה־ when speaking generally, just as English can say Identity theft is a serious fraud without needing the.

In a construct chain, the definite article usually goes on the second noun, not the first.

So:

• גניבת זהות = identity theft
• גניבת הזהות = the identity theft / the theft of the identity

Not:

• הגניבת זהות ← this is not correct

This is a very important rule in Hebrew construct phrases.

In standard Hebrew, with this kind of noun-to-noun sentence, היא is normally kept.

So:

• גניבת זהות היא הונאה רצינית = correct and natural
• גניבת זהות הונאה רצינית = sounds incomplete or ungrammatical in standard Hebrew

Hebrew often omits to be in the present tense with adjectives, but when one noun is being identified as another noun, הוא/היא is usually needed.

Yes. This is the normal, neutral order:

subject + היא/הוא + predicate

So:

• גניבת זהות = subject
• היא = linking word
• הונאה רצינית = predicate

You can sometimes change word order for emphasis, but the given sentence is the most straightforward and standard way to say it.

Yes. Hebrew could also use an adjective-based sentence, for example:

גניבת זהות היא רצינית
But that sounds less natural for the intended meaning.

The original sentence is stronger and more idiomatic because it classifies identity theft as a serious fraud, not just as serious.

So the version you were given is more precise and more natural in formal or explanatory language.