El meu germà diu que l'armari de dalt és ple, però el de sota encara és buit.

My brother says that the upper cupboard is full, but the one below is still empty.

Breakdown of El meu germà diu que l'armari de dalt és ple, però el de sota encara és buit.

• ser: to be
• però: but
• meu: my
• el germà: the brother

Questions & Answers about El meu germà diu que l'armari de dalt és ple, però el de sota encara és buit.

Why is it el meu germà instead of just meu germà?

In standard Catalan, a possessive usually goes together with a definite article:

• el meu germà
• la meva germana
• els meus llibres

That is different from English, where you normally just say my brother without the.

So el meu germà is the normal everyday pattern.
You may also come across forms like mon germà, but those are more literary, regional, or traditional.

Why is it l'armari and not el armari?

Because el becomes l' before a word starting with a vowel or silent h:

• el + armari → l'armari
• el + hotel → l'hotel

This is just the normal apostrophe rule in Catalan.
It works much like French or Italian elision.

What does que do in diu que? Can Catalan leave it out like English sometimes does with that?

Here que introduces the clause after diu:

• El meu germà diu que...
• My brother says that...

In English, you can often omit that.
In Catalan, que is normally kept, so diu que is the standard form.

What do de dalt and de sota mean here?