כשיש הרבה אבק בחדר, אני מתעטשת והאף שלי נעשה אדום.

כשיש is made of two parts:

• כש־ = when
• יש = there is / there are

So כשיש means when there is or when there are.

You can think of כש־ as a shortened form of כאשר, which also means when.

No. יש stays the same for both singular and plural.

• יש אבק = there is dust
• יש הרבה ספרים = there are many books

Unlike English, Hebrew does not change יש depending on whether the following noun is singular or plural.

Because אבק means dust, which is usually treated as an uncountable mass noun, like dust in English.

So Hebrew normally says:

• הרבה אבק = a lot of dust

Even though הרבה often appears with plural count nouns, it can also be used with singular mass nouns like אבק, מים, אוכל, and so on.

In unpointed Hebrew writing, בחדר can mean either one.

• בַּחדר = in the room
• בְּחדר = in a room

Without vowel marks, they are written the same way: בחדר.

So the exact meaning depends on context or pronunciation. This is very common in Hebrew spelling.

In the present tense, Hebrew verb forms show gender and number, but they do not clearly show person.

So מתעטשת could mean:

• I sneeze
• you sneeze when speaking to one woman
• she sneezes

Because of that, Hebrew often includes the subject pronoun in the present tense when it is needed for clarity. So אני מתעטשת clearly means I sneeze.

Because the speaker is feminine.

• אני מתעטש = I sneeze, said by a male speaker
• אני מתעטשת = I sneeze, said by a female speaker

Hebrew present-tense forms agree with the speaker’s gender.

It comes from the infinitive להתעטש, which means to sneeze.

Some present-tense forms are:

• מתעטש = masculine singular
• מתעטשת = feminine singular
• מתעטשים = masculine plural / mixed plural
• מתעטשות = feminine plural

So מתעטשת is simply the feminine singular present form of להתעטש.

This is the normal Modern Hebrew way to say my nose:

• האף שלי = my nose

Hebrew usually places שלי and the other של forms after the noun:

• הספר שלי = my book
• הבית שלך = your house
• הילדים שלהם = their children

So האף שלי is the standard word order.

Here נעשה means becomes, gets, or turns.

So:

• האף שלי נעשה אדום = my nose becomes red / gets red / turns red

It describes a change of state.

This is a common Hebrew way to say that something changes into a new condition.

Because אף is a masculine singular noun.

In Hebrew, both verbs and adjectives often agree with the noun in gender and number.

So you get:

• האף שלי נעשה אדום

If the noun were feminine, the forms would change. For example, with עין:

• העין שלי נעשית אדומה = my eye becomes red

So the masculine forms here are caused by אף being masculine.

Yes. That is very natural, especially in everyday spoken Hebrew.

• האף שלי נעשה אדום
• האף שלי נהיה אדום

Both mean my nose gets red.

A rough difference:

• נהיה often sounds more everyday and conversational
• נעשה can sound a little more formal, written, or stylistic

But both are perfectly understandable.

Because כשיש הרבה אבק בחדר is an introductory time clause: when there is a lot of dust in the room.

After that, the main clause begins:

• אני מתעטשת והאף שלי נעשה אדום

So the comma helps separate:

• the when part
• from the main statement

This works very much like English punctuation in sentences such as When there is a lot of dust in the room, I sneeze...

Because כש means when, while אם means if.

Here the sentence describes something that happens regularly or whenever that situation occurs:

• כשיש הרבה אבק... = when there is a lot of dust...
• אם יש הרבה אבק... = if there is a lot of dust...

Using אם would make it sound more like a condition or possibility. Using כש makes it sound like a usual repeated result, which fits this sentence better.