המורה אומרת שלא צריך למחוק כל תרגיל, כי אחר כך אפשר להבין מה לא טוב בו.

Because המורה can mean either teacher (male) or teacher (female). The verb tells you which one is meant here.

• המורה אומרת = the teacher is female
• המורה אומר = the teacher is male

So the sentence is specifically talking about a female teacher.

שלא is made of:

• ש־ = that
• לא = not

So אומרת שלא צריך... means says that there is no need to... or says that you don’t need to...

This is very common after verbs like say, think, know, etc.

Here צריך is being used impersonally.

Hebrew often uses masculine singular forms in general statements where English might use:

• you
• one
• people
• it is necessary

So לא צריך למחוק means something like:

• you don’t need to erase
• there’s no need to erase
• one doesn’t need to erase

It does not have to refer to a specific male person. This masculine singular form is often the default in impersonal expressions.

Because it comes after צריך.

In Hebrew, words like צריך are commonly followed by an infinitive:

• צריך לעשות = need to do
• אפשר להבין = can understand / it is possible to understand
• רוצה ללכת = wants to go

So:

• צריך למחוק = need to erase

Because כל תרגיל here is not treated as a definite direct object.

In Hebrew, את is usually used before a definite direct object, such as:

• את התרגיל = the exercise
• את הספר = the book

But here we have:

• כל תרגיל = every exercise / each exercise

That is not the same as the exercise, so את is not used.

Compare:

• למחוק את התרגיל = erase the exercise
• למחוק כל תרגיל = erase every exercise

כל תרגיל means every exercise or each exercise.

It refers to the exercises one by one.

Compare:

• כל תרגיל = every/each exercise
• כל התרגילים = all the exercises

That distinction is useful:

• כל תרגיל focuses on each individual item
• כל התרגילים refers to the whole set

כי means because, and אחר כך means afterward / later.

So כי אחר כך... introduces the reason:

• because later...
• because afterward...

This explains why the teacher says not to erase every exercise.

Also, אחר כך is normally written as two words.

Because אפשר is another very common impersonal structure in Hebrew.

• אפשר = it is possible / one can / you can

So:

• אפשר להבין = it’s possible to understand / you can understand

Just like צריך, this does not need a specific subject. English often uses you in these general statements, but Hebrew often prefers an impersonal form.

Literally, it is something like:

• what is not good in it
• what is not good about it

In natural English, this often becomes:

• what is wrong with it
• what isn’t good about it

A key point is that מה here introduces an indirect question:

• להבין מה לא טוב בו = to understand what is wrong with it

So it is not asking a direct question out loud; it is embedded inside the sentence.

בו is made of:

• ב־ = in / on / with / about
• ו = him / it

So בו can mean in him, in it, about it, depending on context.

Here it refers back to תרגיל, which is a masculine singular noun. So בו means something like:

• in it
• about it
• with it

In this sentence, the idea is what is wrong with the exercise.

If the noun were feminine, Hebrew would normally use בה instead.

It can mean either one, depending on context.

Hebrew present tense often covers both:

• says
• is saying

So המורה אומרת could be understood as:

• the teacher says
• the teacher is saying

In a sentence like this, English usually uses says.

No. They are different.

• לא צריך = there’s no need to / you don’t need to
• אסור = it is forbidden / you must not

So this sentence is not giving a strict prohibition. It is saying that erasing every exercise is unnecessary, not that it is forbidden. The tone is much softer.