Treu la motxilla de sota el llit.

Treu comes from the verb treure, which means to take out, to remove, or to pull out.

In this sentence, treu is the informal singular imperative: it is a command addressed to one person you would say tu to.

So here it means something like take out / remove / get.

Catalan often leaves out subject pronouns when they are understood from the verb form.

So instead of saying Tu treu..., Catalan normally just says Treu....

That is very natural. The verb ending already tells you who the command is aimed at.

Because motxilla is a specific noun here, and Catalan normally uses the definite article with it: la motxilla = the backpack.

Also, motxilla is a feminine singular noun, so it takes la.

You should learn nouns together with their article when possible:

• la motxilla
• el llit

De sota means from under or from underneath.

A useful comparison is:

• sota el llit = under the bed
• de sota el llit = from under the bed

So in this sentence, the backpack is located under the bed, and the action is to take it from that place.

Because the de belongs to the expression de sota, and el llit comes after sota.

The structure is:

de + sota + el llit

Catalan only contracts de + el into del when those two words are directly next to each other. Here they are not, so you keep el llit.

So:

• del llit = from de + el llit
• de sota el llit = from under the bed

They are related, but not exactly the same in use.

With a noun phrase, Catalan very often just uses sota:

• sota el llit = under the bed

You may also hear sota de in some contexts or varieties, but in this sentence de sota is not the same thing as sota de. Here de means from, so de sota el llit means from under the bed.

Yes. That is very natural if the backpack is already understood from context.

• Treu la motxilla de sota el llit. = Take the backpack from under the bed.
• Treu-la de sota el llit. = Take it from under the bed.

In Catalan, with an affirmative command, the object pronoun is usually attached to the end of the verb with a hyphen:

• treu-la

The command changes depending on who you are addressing:

• Treu la motxilla de sota el llit. = informal singular (tu)
• Tregui la motxilla de sota el llit. = formal singular (vostè)
• Traieu la motxilla de sota el llit. = informal plural (vosaltres)
• Treguin la motxilla de sota el llit. = formal plural (vostès)

So treu is specifically the tu form.

The order is:

Treu + la motxilla + de sota el llit

That is:

• verb
• direct object
• place/source phrase

This is a very normal order for commands in Catalan. It matches English quite closely in this kind of sentence.

Because treure emphasizes removing something from a place.

• treure = take out, remove
• agafar = take, grab, pick up

If the idea is specifically get the backpack from under the bed, treure is a very good choice because it highlights that the backpack is being taken out from where it is.