رمل القطط في صندوق فضلات القطط.

Because رمل القطط is an iḍāfa construction, often called a noun + noun or possessive structure.

In an iḍāfa:

• the first noun does not take ال
• the second noun can make the whole phrase definite

So:

• رمل = sand / litter
• القطط = the cats

Together, رمل القطط means cat litter or literally the cats' sand/litter.

You cannot normally say الرمل القطط for this meaning.

القطط means cats.

Arabic often uses a plural noun when English uses a noun as a classifier, like:

• cat litter
• dog food
• children's clothes

So رمل القطط literally looks like the litter of cats, but in natural English it is just cat litter.

The plural is normal here because the phrase refers to cats in general, not one specific cat.

It appears in two different noun phrases:

• رمل القطط = cat litter
• فضلات القطط = cat waste / cat droppings

Then the second phrase is inside a bigger one:

• صندوق فضلات القطط = the cat litter box / the box for cat waste

So the full expression is built from smaller parts. Arabic often repeats the noun rather than replacing it with something shorter.

This is another iḍāfa chain.

Break it down like this:

• صندوق = box
• فضلات = waste / droppings
• القطط = the cats

So:

• فضلات القطط = cat waste
• صندوق فضلات القطط = box of cat waste, meaning cat litter box

A useful rule:

• in a chain like this, all the nouns before the last one are in construct form
• that is why neither صندوق nor فضلات has ال

Because it is inside an iḍāfa chain.

In صندوق فضلات القطط:

• صندوق is connected to فضلات
• فضلات is connected to القطط

When a noun is the first part of an iḍāfa, it normally does not take ال.

So even though فضلات looks indefinite by itself, the whole phrase becomes definite through the final definite noun القطط.

By itself, رمل القطط في صندوق فضلات القطط is usually understood as a phrase, not a full formal sentence with a verb.

It can work as:

• a label
• a caption
• a short instruction
• part of a longer sentence

Literally it is something like:

• cat litter in the cat litter box

If you wanted a full sentence, Arabic might say something like:

• يوجد رمل القطط في صندوق فضلات القطط = The cat litter is in the litter box.

But Arabic often leaves out words in labels and simple descriptions.

في means in or inside.

So it introduces the location:

• في صندوق فضلات القطط = in the cat litter box

Also, after a preposition like في, the following noun is grammatically in the genitive case. In normal unvocalized writing, you usually do not see that case ending, but it is there in full grammar.

They are not the same thing.

• رمل = sand, and by extension litter
• فضلات = waste, droppings, excrement

So:

• رمل القطط = cat litter
• فضلات القطط = cat waste

That is why the sentence uses both words:

• one for the material in the box
• one for what the box is meant to collect

A careful reading with case endings would be approximately:

ramlu l-qiṭaṭi fī ṣundūqi faḍalāti l-qiṭaṭi

A simpler learner-friendly pronunciation without worrying much about case endings is:

raml al-qiṭaṭ fī ṣundūq faḍalāt al-qiṭaṭ

A few pronunciation notes:

• ق is a deep q sound
• ط is an emphatic t
• ض is an emphatic d-like sound
• صندوق is pronounced roughly ṣundūq

Arabic starts with the main thing being talked about:

• رمل القطط = cat litter

Then it adds the location:

• في صندوق فضلات القطط = in the litter box

So the order is:

• thing + location

This is very natural in Arabic, especially in short descriptions and labels.

It could, but the meaning would change.

For example:

• رمل القطة would sound like the litter of the female cat
• رمل القط would sound like the litter of the male cat or one particular cat

But رمل القطط means cat litter in a general sense, referring to cats as a class. That is why the plural is the most natural choice here.