אני לא יכולה להבין את המשפט הזה בשנייה אחת.

Because יכולה is the feminine singular form of יכול/יכולה, meaning able to or can.

So this sentence is being said by a female speaker:

• אני לא יכולה... = I cannot...

If the speaker were male, it would be:

• אני לא יכול להבין את המשפט הזה בשנייה אחת.

Hebrew often marks gender in the present tense, so this is something English speakers usually notice quickly.

Not always, but it is very common.

Hebrew can sometimes omit subject pronouns, especially when the meaning is clear from context. However, in the present tense, forms like יכולה tell you gender and number, but not always person as clearly as English does. Because of that, speakers often include אני for clarity.

So both of these can work:

• אני לא יכולה להבין את המשפט הזה בשנייה אחת
• לא יכולה להבין את המשפט הזה בשנייה אחת

The version with אני is clearer and more complete, especially for learners.

The word לא means not.

In this sentence, it comes before יכולה:

• אני לא יכולה... = I cannot...

Unlike English, Hebrew does not need a helping verb like do:

• English: I do not understand
• Hebrew: אני לא מבינה

And with can:

• English: I cannot understand
• Hebrew: אני לא יכולה להבין

So the basic pattern is simply:

• subject + לא
  • verb/expression

Literally, it means am able to understand.

Hebrew commonly expresses can with יכול/יכולה plus an infinitive:

• יכולה = able, can
• להבין = to understand

So:

• אני יכולה להבין = I can understand
• אני לא יכולה להבין = I cannot understand

This is very close to saying I am able to understand, but in normal English we usually translate it as I can understand.

Because להבין is the infinitive, meaning to understand.

After יכול/יכולה, Hebrew normally uses an infinitive:

• יכול ללכת = can go
• יכולה לקרוא = can read
• יכולה להבין = can understand

The ל at the beginning is the usual marker of the infinitive in Hebrew, often corresponding to English to.

So:

• להבין = to understand

את is the marker of a definite direct object.

It does not really get translated into English, but it appears before a direct object that is definite, such as:

• a noun with ה
• a name
• a pronoun
• something like this/that

Here the direct object is המשפט הזה = this sentence, which is definite, so Hebrew uses את:

• להבין את המשפט הזה = to understand this sentence

Compare:

• אני קוראת ספר = I am reading a book
• אני קוראת את הספר = I am reading the book

English has no equivalent word, so this often feels strange at first.

Because המשפט הזה means this sentence, while זה המשפט usually means this is the sentence.

These are two different structures:

• המשפט הזה = this sentence
• זה המשפט = this is the sentence

In Hebrew, when this directly describes a noun, the usual pattern is:

• noun + demonstrative

So:

• הספר הזה = this book
• הילד הזה = this boy
• המשפט הזה = this sentence

That word order is different from English, where this comes before the noun.

That is the normal Hebrew word order for demonstratives like this and that when they modify a noun.

English:

• this sentence

Hebrew:

• המשפט הזה

So Hebrew puts the noun first, then this:

• הבית הזה = this house
• החברה הזאת = this friend / this company
• המשפט הזה = this sentence

This is one of the most common word-order differences between English and Hebrew.

Because in Hebrew, a noun modified by this is definite, so it usually takes the definite article ה.

So Hebrew says:

• המשפט הזה

Not:

• משפט הזה

This is similar to:

• הספר הזה = this book
• הילדה הזאת = this girl

Even though English does not use the together with this, Hebrew does mark the noun as definite in this structure.

It literally means in one second, but in natural English it often means something like:

• in a second
• instantly
• all at once
• immediately

So the sentence is saying that the speaker cannot understand the sentence instantly or all in one go.

Breaking it down:

• ב־ = in
• שנייה = second
• אחת = one

So:

• בשנייה אחת = in one second / in a single second

Because שנייה is a feminine noun, and the number one must agree with it in gender.

• אחד = one, masculine
• אחת = one, feminine

Since שנייה is feminine, Hebrew uses:

• שנייה אחת

Compare:

• יום אחד = one day
• ספר אחד = one book
• שעה אחת = one hour
• מילה אחת = one word

So אחת is correct here because שנייה is feminine.

In modern Hebrew, the number one often comes after the noun:

• יום אחד = one day
• שעה אחת = one hour
• שנייה אחת = one second

This is different from English, where one usually comes before the noun.

So even though English says:

• one second

Hebrew commonly says:

• שנייה אחת

That word order is completely normal.

You could, but it would not mean exactly the same thing.

• אני לא יכולה להבין... = I cannot understand...
• אני לא מבינה... = I do not understand...

In your sentence, לא יכולה להבין emphasizes inability to do it that quickly or all at once. It suggests:

• I cannot process this sentence in a single second
• I need more time

If you say:

• אני לא מבינה את המשפט הזה

that simply means:

• I do not understand this sentence

So לא יכולה להבין fits better when the point is about speed or difficulty in understanding immediately.

Yes, it is understandable and natural enough, especially if the speaker means:

• I cannot understand this sentence instantly
• I cannot grasp this sentence in a single second

A native speaker might also say similar things such as:

• אני לא יכולה להבין את המשפט הזה מיד
• אני לא יכולה להבין את המשפט הזה תוך שנייה
• אני לא מבינה את המשפט הזה כל כך מהר

But your sentence is perfectly fine and clearly expresses the idea.