על השולחן יש שתי פרוסות לחם, חמאה וריבה.

Because Hebrew very often puts the location first in this kind of sentence.

So על השולחן יש... is a very natural pattern meaning On the table there is/are...

It sets the scene first, then tells you what is there.
You could also say יש ... על השולחן, but על השולחן יש... sounds especially natural when you are introducing what is in a place.

יש means there is or there are.

So:

• יש ספר = there is a book
• יש ספרים = there are books

In this sentence, יש introduces the things that are on the table.

Because יש does not normally change for singular vs. plural in everyday Modern Hebrew.

So Hebrew says:

• יש תפוח = there is an apple
• יש תפוחים = there are apples

The same word, יש, works for both.

You may sometimes see ישנם or ישנן in more formal Hebrew, but in normal speech יש is the standard choice.

Because על stays a separate word.

Hebrew prepositions like ב, ל, and כ often combine with ה:

• ב + ה → בַּ
• ל + ה → לַ
• כ + ה → כַּ

But על does not combine that way. So you simply say:

• על השולחן = on the table

Without ה, על שולחן would mean on a table, not on the table.

Because פרוסה is a feminine noun.

In Hebrew, the number 2 has different forms depending on gender when it comes before a noun:

• שני for masculine nouns
• שתי for feminine nouns

Since the singular is פרוסה, the correct form is:

• שתי פרוסות = two slices

Also, שתיים is the standalone form, used when the number appears by itself rather than directly before a noun.

Because in Hebrew, nouns counted by 2–10 are normally plural.

So:

• שתי פרוסות = two slices
• not שתי פרוסה

That part is actually similar to English, where we also say two slices, not two slice.

Because Hebrew usually expresses X of Y with a compact noun-noun structure rather than with של.

So:

• פרוסות לחם = slices of bread

This is the normal Hebrew way to say it.

Using של here would sound much less natural. Hebrew strongly prefers the shorter noun chain in this kind of expression.

Because they are indefinite here.

The sentence is talking about:

• two slices of bread
• butter
• jam

not specifically the bread, the butter, and the jam.

Also, חמאה and ריבה are being used like mass nouns, similar to English butter and jam, which often appear without the when speaking generally.

If the bread were definite, Hebrew could show that, for example:

• פרוסות הלחם = slices of the bread

As written, the most natural reading is that these are separate items in a list:

• two slices of bread
• butter
• jam

all on the table.

If you wanted to say that the bread has butter and jam on it, Hebrew would normally make that clearer, for example:

• שתי פרוסות לחם עם חמאה וריבה = two slices of bread with butter and jam

So the original sentence sounds more like a list of things present on the table.

Because פרוסות לחם is one complete item in a list, and then חמאה and ריבה are additional items.

So the structure is:

• שתי פרוסות לחם
• חמאה
• ריבה

The comma separates list items, much like in English.

Also notice that ו appears before the last item, which is normal Hebrew list style:

• לחם, חמאה וריבה = bread, butter and jam