במקום שהיא משאירה את הערימה על הרצפה, היא ממיינת אותה.

Here, במקום means instead of or rather than.

Even though מקום literally means place, the expression במקום ש... is a very common way to say instead of ... doing something.

So in this sentence, במקום שהיא משאירה... means instead of her leaving...

Because the sentence is using a subordinate clause after במקום.

• ש means that or introduces a clause
• היא means she
• together: שהיא = that she / she in a clause

So:

• במקום שהיא משאירה... = instead of her leaving...

Also, in Hebrew, ש is usually attached directly to the next word, so ש + היא becomes שהיא.

Yes. In fact, many learners will hear this kind of sentence more often with an infinitive:

• במקום להשאיר את הערימה על הרצפה, היא ממיינת אותה.

That means the same thing: Instead of leaving the pile on the floor, she sorts it.

So both patterns exist:

• במקום שהיא משאירה...
• במקום להשאיר...

The infinitive version is often more compact and very common.

Because the subject is היא, meaning she.

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

• משאיר = masculine singular
• משאירה = feminine singular
• משאירים = masculine plural
• משאירות = feminine plural

Since the subject is she, the sentence uses משאירה.

For the same reason: its subject is also היא.

• היא ממיינת = she sorts
• הוא ממיין = he sorts

In this sentence, both verbs refer to the same feminine singular subject, so both appear in feminine singular form:

• היא משאירה
• היא ממיינת

Because הערימה is a definite direct object.

In Hebrew, את marks a direct object when that object is definite. Definite usually means it has ה־ on it, or is a name, or is otherwise specific.

Here:

• הערימה = the pile
• since it is definite, Hebrew uses את
• משאירה את הערימה = leaves the pile

Compare:

• היא משאירה ערימה = she leaves a pile
  no את, because it is indefinite
• היא משאירה את הערימה = she leaves the pile

אותה means her or it as a feminine singular direct object.

Here it refers back to הערימה, which is a feminine noun. So Hebrew uses the feminine object pronoun:

• אותו = him / it masculine
• אותה = her / it feminine

Since ערימה is feminine, the sentence says:

• היא ממיינת אותה = she sorts it

Even though English uses it, Hebrew still has to choose masculine or feminine based on the noun.

Because על means on.

So:

• על = on
• רצפה = floor
• הרצפה = the floor

Together:

• על הרצפה = on the floor

This is a very standard preposition choice in Hebrew, just like in English.

Hebrew often keeps the subject pronoun in present-tense sentences because the present-tense verb form does not clearly show person by itself.

For example:

• ממיינת tells you feminine singular
• but it does not by itself tell you whether the subject is I, you, or she, depending on context

So היא ממיינת clearly means she sorts.

The second היא is therefore very natural and helps keep the sentence clear.

ממיינת is the feminine singular present-tense form of למיין, which means to sort, to classify, or to organize by categories.

So:

• למיין = to sort
• הוא ממיין = he sorts
• היא ממיינת = she sorts

This verb is often used for sorting objects, documents, laundry, files, and so on.

The dictionary form is להשאיר, which means to leave or to leave behind.

Its present forms are:

• משאיר = masculine singular
• משאירה = feminine singular
• משאירים = masculine plural
• משאירות = feminine plural

So:

• היא משאירה את הערימה על הרצפה = she leaves the pile on the floor

Yes, somewhat.

The given sentence starts with the contrast:

• במקום שהיא משאירה את הערימה על הרצפה, היא ממיינת אותה.

This highlights instead of leaving it on the floor...

But Hebrew could also express the same idea in slightly different ways, for example:

• היא ממיינת אותה במקום להשאיר את הערימה על הרצפה.

That said, the original order is very natural if the speaker wants to emphasize the contrast between the two actions.

Yes, it is understandable and grammatical.

However, many speakers might more naturally say:

• במקום להשאיר את הערימה על הרצפה, היא ממיינת אותה.

Why? Because Hebrew often prefers the infinitive after במקום when the subject stays the same.

So the original sentence is fine, but the infinitive version may sound a bit smoother in everyday usage.