אני מורחת חמאה על פרוסה אחת וריבה על הפרוסה השנייה.

Because the speaker is female.

In Hebrew present tense, the verb form agrees with gender and number:

• מורח = masculine singular
• מורחת = feminine singular
• מורחים = masculine plural / mixed plural
• מורחות = feminine plural

So:

• אני מורחת = a woman saying I spread / I am spreading
• אני מורח = a man saying the same thing

It can mean either one, depending on context.

Hebrew present tense often covers both:

• simple present: I spread
• present continuous: I am spreading

So אני מורחת חמאה... could describe:

• a general action
• what someone is doing right now

Hebrew does not usually need a separate word like am in this kind of sentence.

Because in Hebrew, the present tense usually does not use a separate verb meaning to be.

English says:

• I am spreading

Hebrew simply says:

• אני מורחת

That already includes the idea of I am spreading or I spread.

This is very normal in Hebrew. The verb form itself carries the present-tense meaning.

Because את is used before a definite direct object.

Here, חמאה and ריבה are not definite. They mean butter and jam in a general sense, not the butter or the jam.

So:

• אני מורחת חמאה = I spread butter
• אני מורחת את החמאה = I spread the butter

The same applies to ריבה.

Because the pattern with למרוח is:

• למרוח משהו על משהו
• to spread something on something

So in the sentence:

• חמאה = what is being spread
• על פרוסה אחת = where it is being spread

That is why על is the natural preposition here.

Because in Hebrew, the number one usually comes after the noun.

So:

• פרוסה אחת = one slice

Also, פרוסה is a feminine noun, so the number must also be feminine:

• masculine: אחד
• feminine: אחת

That is why it is פרוסה אחת.

Because it describes פרוסה, and פרוסה is feminine.

In Hebrew, adjectives and ordinal numbers agree with the noun in:

• gender
• number
• definiteness

So:

• פרוסה שנייה = a second slice
• הפרוסה השנייה = the second slice

If the noun were masculine, you would use השני instead.

Because in Hebrew, when a noun is definite and is followed by an adjective or ordinal, both usually become definite.

So:

• פרוסה שנייה = a second slice
• הפרוסה השנייה = the second slice

This is a very important Hebrew pattern:

• הילד הטוב = the good boy
• הספר החדש = the new book
• הפרוסה השנייה = the second slice

Because they are functioning differently.

• פרוסה אחת introduces one slice
• הפרוסה השנייה refers to the other specific slice

In English, this contrast is natural too:

• on one slice
• on the second slice

The first phrase is indefinite; the second is definite because it points to a clearly identified item.

Because Hebrew often leaves out a repeated verb when it is easy to understand.

The full version could be:

• אני מורחת חמאה על פרוסה אחת ומורחת ריבה על הפרוסה השנייה

But Hebrew very naturally shortens this to:

• אני מורחת חמאה על פרוסה אחת וריבה על הפרוסה השנייה

English does something similar:

• I spread butter on one slice and jam on the second

The missing verb is understood from the first part.

Literally, it means one slice.

In this sentence, though, it also helps create a contrast with the second slice, so one slice is the best understanding.

If you just wanted to say a slice with no contrast, Hebrew might use a different phrasing depending on context. Here, אחת is meaningful because the sentence is dividing things between two slices.

The sentence is organized like this:

• אני מורחת = I spread / I am spreading
• חמאה = butter
• על פרוסה אחת = on one slice
• וריבה על הפרוסה השנייה = and jam on the second slice

So the pattern is roughly:

• subject + verb + thing spread + על + surface

Then the second half omits the repeated verb.

This order is very natural in Hebrew for this kind of sentence.