لم تعد القطة تختبئ خلف حاملة القطط أو تموء.

In this sentence, لم تعد is a very common way to say no longer or not anymore.

It comes from the verb عاد / يعود, which literally means to return.
So لم تعد تختبئ is literally something like she did not return to hiding, but in natural English it means she no longer hides.

Because the subject is القطة (the cat), the verb is in the feminine singular form: تعد.

So:

• لم يعد = he/it no longer
• لم تعد = she/it no longer

Here it is clearly she/it no longer, because of القطة.

Because لم makes the following imperfect verb jussive.

The normal imperfect form is:

• تعودُ = she returns

After لم, it becomes:

• تعدْ

This happens because عاد is a hollow verb (a weak verb with a long vowel in the middle), and the long vowel drops in the jussive.

So:

• تعودُ → normal imperfect
• لم تعدْ → jussive after لم

That is completely normal Arabic grammar.

Because لا and لم تعد do different jobs.

• لا تختبئ = it does not hide
• لم تعد تختبئ = it no longer hides

So لم تعد adds the idea of a change of behavior. It implies that earlier, the cat used to hide, but now that is no longer true.

That is why لم تعد is the right choice if the meaning is not anymore rather than simple present negation.

Because Arabic often uses verb–subject word order, especially in straightforward narrative sentences.

So this structure is very normal:

• لم تعد القطة تختبئ...

Literally: No longer did the cat hide...

The noun القطة is still the subject, even though it comes after the verb.

If fully vowelled, it would be القطةُ, showing that it is nominative as the subject.

Because both verbs refer back to the same subject: القطة.

Since القطة is a feminine singular noun, the verbs that describe its actions are also feminine singular:

• تختبئ = she hides
• تموء = she meows

This is standard agreement in Arabic.

Because after عاد in this kind of expression, Arabic normally uses a bare imperfect verb directly.

So you say:

• لم تعد تختبئ
• not لم تعد أن تختبئ

The same applies when another verb is coordinated after it:

• لم تعد ... تختبئ ... أو تموء

So the sentence is natural as written. Adding أن here would sound unnatural in standard usage.

Yes. In this sentence, لم تعد governs the whole coordinated action:

• تختبئ خلف حاملة القطط
• أو تموء

So the meaning is that the cat no longer does either of those things.

In natural English, that can come out as:

• The cat no longer hides behind the carrier or meows
• or more explicitly: The cat neither hides behind the carrier nor meows anymore

Yes, أو normally means or.

In a negative sentence like this, English sometimes prefers or, and sometimes we paraphrase with nor. Arabic can still use أو naturally.

So here أو تموء means:

• or meows

But because the whole sentence is negative, the overall sense is:

• the cat no longer hides behind the carrier or meows
• in other words, it does neither

So أو is not strange here.

Because حاملة القطط is an iḍāfa construction.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun determines whether the whole phrase is definite

So:

• حاملة القطط = literally carrier of the cats / cats-carrier
• as a natural phrase: the cat carrier

Because القطط is definite, the whole expression is definite.

That is why الحاملة القطط would be wrong here.

Because حاملة القطط is being used like the name of an object type: cat carrier.

Arabic often does this in compound-like expressions. The plural can describe the general purpose or class of the object, not the number of animals actually involved in this moment.

So حاملة القطط means something like:

• a carrier for cats
• cat carrier

If you said حاملة القطة, that would sound more like the carrier of the cat or a carrier associated with one specific cat.

خلف means behind.

It is often treated as a spatial noun used in a construction like:

• خلفَ + noun

So:

• خلف حاملة القطط = behind the cat carrier

The noun after خلف is in the genitive:

• خلفَ حاملةِ القططِ

In normal unvowelled writing, you usually do not see those endings, but that is the grammar behind the phrase.

They come from these verbs:

• اختبأ / يختبئ = to hide
• ماء / يموء = to meow

So:

• تختبئ = she hides
• تموء = she meows

Both are in the imperfect, feminine singular, agreeing with القطة.

A careful pronunciation would be roughly:

• تختبئ = takhtabiʾu
• تموء = tamūʾu

The final hamza in both words is real and should be pronounced.

That final hamza is why the spelling may look unusual to an English-speaking learner, but it is completely normal Arabic spelling.