אם את לא מבינה מילה, כדאי להיעזר במילון ולא לנחש.

Here it is the pronoun את = you (addressing one female).

Hebrew writes both words the same way in unpointed text:

• אַתְּ = you (feminine singular)
• אֶת = the direct object marker

In this sentence, it must be you, because it is followed by לא מבינה = do not understand in the feminine singular.

So:

• אם את לא מבינה... = If you (f.s.) don’t understand...

Not the object marker.

Both words show that the sentence is speaking to one female.

• את = you (feminine singular)
• מבינה = understand, feminine singular present form

If you were speaking to a man, you would say:

• אם אתה לא מבין מילה, כדאי להיעזר במילון ולא לנחש.

If you were speaking to more than one person, the sentence would change again.

The dictionary form is להבין = to understand.

מבינה is the present tense feminine singular form.

Related forms:

• אני מבין / מבינה = I understand
• אתה מבין = you understand (m.s.)
• את מבינה = you understand (f.s.)
• הוא מבין = he understands
• היא מבינה = she understands

So in this sentence, את לא מבינה literally means you are not understanding / you do not understand, and in natural English it is simply you don’t understand.

אם means if.

It introduces a condition:

• אם את לא מבינה מילה... = If you don’t understand a word...

So the sentence has this structure:

• condition: if you don’t understand a word
• advice: it’s advisable to use a dictionary and not guess

This is very normal Hebrew.

לא ... מילה literally means not ... a word, but in English we often translate that as not ... a word or not ... any word, depending on context.

So:

• לא מבינה מילה = don’t understand a word
• idiomatically: don’t understand even one word / don’t understand any word

Hebrew often uses a singular noun like this after a negative statement.

Because מילה here is indefinite: a word, not the word.

The direct object marker את is generally used before definite direct objects.

Compare:

• אני מבינה מילה = I understand a word
• אני מבינה את המילה = I understand the word
• אני מבינה את המילה הזאת = I understand this word

So in your sentence, מילה is indefinite, so there is no object marker.

כדאי means something like:

• it’s advisable
• it’s a good idea
• it’s worth
• better to

In this sentence:

• כדאי להיעזר במילון = it’s advisable to use a dictionary

A very common Hebrew pattern is:

• כדאי + infinitive

For example:

• כדאי ללמוד = it’s כדאי to study / it’s a good idea to study
• כדאי לבדוק = it’s a good idea to check
• כדאי לחכות = it’s better to wait

Notice that כדאי does not change here for gender or number. It is used impersonally.

להיעזר means to make use of, to get help from, or to rely on something or someone for help.

In this sentence, להיעזר במילון means:

• to use a dictionary
• more literally, to get help from a dictionary

This verb is a little more nuanced than just a plain to use. It suggests using something as an aid or support.

So:

• להיעזר במילון = to consult a dictionary / use a dictionary for help

The ב־ means in / with / by means of, depending on context.

With להיעזר, Hebrew usually uses ב־ before the thing you are getting help from:

• להיעזר במילון = to use a dictionary / get help from a dictionary
• להיעזר בחבר = to get help from a friend
• להיעזר במורה = to get help from a teacher

So here במילון is the natural prepositional phrase after להיעזר.

In unpointed Hebrew, במילון could represent either:

• in a dictionary
• in the dictionary

But in this sentence, the meaning is simply use a dictionary / consult a dictionary.

Because Hebrew often avoids repeating words when the meaning is already clear.

So:

• כדאי להיעזר במילון ולא לנחש

literally means:

• It is advisable to use a dictionary and not to guess

Hebrew does not need to repeat כדאי before לנחש.

A more expanded version would be:

• כדאי להיעזר במילון ולא כדאי לנחש

But that sounds heavier and less natural here.

The dictionary form is לנחש = to guess.

In this sentence:

• ולא לנחש = and not to guess

So the advice is:

• use a dictionary
• don’t guess

It is an infinitive, just like להיעזר.

Because they both depend on כדאי.

As mentioned above, Hebrew commonly uses:

• כדאי + infinitive

So:

• כדאי להיעזר... = it’s advisable to use...
• ולא לנחש = and not to guess

This is the same basic structure as English:

• It’s better to use a dictionary and not to guess.

Because the sentence begins with a conditional clause:

• אם את לא מבינה מילה = If you don’t understand a word

Then comes the main clause:

• כדאי להיעזר במילון ולא לנחש = it’s advisable to use a dictionary and not guess

In English, we also usually put a comma after a starting if clause:

• If you don’t understand a word, use a dictionary instead of guessing.

So the punctuation works similarly.

Yes, that would be grammatical.

• אם את לא מבינה מילה, לא כדאי לנחש. = If you don’t understand a word, it’s not a good idea to guess.

But it is slightly different in emphasis.

Your original sentence:

• כדאי להיעזר במילון ולא לנחש

emphasizes the recommended action first: use a dictionary.

The version with לא כדאי לנחש emphasizes the bad idea: guessing.

Both are natural, but the original gives fuller guidance.