אני לא מבינה מה כתוב בהם.

Because the speaker is female.

In Hebrew, present-tense verbs agree with the subject in gender and number.
So:

• אני מבינה = a woman saying I understand
• אני מבין = a man saying I understand

So אני לא מבינה tells you that the speaker is feminine singular.

מבינה is the present-tense form of the verb להבין (to understand).

In Hebrew, present-tense verb forms behave a bit like participles, and they change for gender and number:

• מבין = masculine singular
• מבינה = feminine singular
• מבינים = masculine plural / mixed plural
• מבינות = feminine plural

So in this sentence, מבינה matches אני when the speaker is female.

Yes, Hebrew can often omit the subject pronoun when it is understood from the verb form.

So both of these are possible:

• אני לא מבינה מה כתוב בהם
• לא מבינה מה כתוב בהם

Including אני can make the sentence a little clearer, more explicit, or slightly more emphatic. In everyday speech, dropping it is very common when the context already makes the subject obvious.

לא is the normal way to negate a verb in Hebrew.

So:

• אני מבינה = I understand
• אני לא מבינה = I do not understand

This is the standard negation pattern for present, past, and future verbal clauses in Hebrew.

Here מה means what, but not as a direct question by itself. It introduces an embedded question: what is written in them.

So the structure is basically:

• אני לא מבינה = I do not understand
• מה כתוב בהם = what is written in them

Together, the second part is the thing the speaker does not understand.

Because Hebrew often uses כתוב as an adjective/passive participle meaning written.

So where English says:

• what is written in them

Hebrew says more literally:

• what written in-them

In present-tense Hebrew, the verb to be is usually omitted, so כתוב can naturally mean is written in context.

Because כתוב agrees with מה, not with בהם.

A learner might look at בהם (in them) and expect something plural, but בהם is just the object of the preposition ב־ (in). It does not control the agreement of כתוב.

The word מה is treated here like a singular item, so כתוב appears in the masculine singular form.

בהם means in them.

It is made from:

• ב־ = in
• הם = they / them (masculine plural)

Combined, they become:

• בהם = in them

This refers to masculine plural nouns, or sometimes a mixed-gender plural group. If the noun were feminine plural, you would usually expect בהן instead.

It can refer to any masculine plural noun, depending on context. For example, it could refer to:

• documents
• books
• messages
• forms
• notebooks

The sentence itself does not tell you what them refers to; that has to come from the surrounding context.

אני לא מבינה מה כתוב בהם is a very natural and standard word order.

Hebrew is somewhat flexible, but this version is the most straightforward for neutral speech. Other word orders are possible for emphasis, for example if you wanted to highlight what is written in them, but the given order is the normal one a learner should be comfortable with first.

A male speaker would say:

אני לא מבין מה כתוב בהם.

The only change is:

• מבינה → מבין

Everything else stays the same.