El portaequipajes lleva la mochila.

Because portaequipajes is one of those Spanish nouns whose singular and plural have the same form.

• el portaequipajes = the luggage rack / baggage carrier
• los portaequipajes = the luggage racks / baggage carriers

This happens with some compound nouns, especially ones built from a verb form plus a noun. So even though it looks plural, el portaequipajes is perfectly singular here.

It is a compound word:

• porta- comes from portar, meaning to carry
• equipajes means luggage

So literally it is something like luggage-carrier or thing that carries luggage.

In real use, it can refer to a luggage rack, roof rack, baggage carrier, or similar structure, depending on context.

In Spanish, articles are used much more often than in English. So Spanish usually says:

• el portaequipajes
• la mochila

rather than dropping the articles.

Without the articles, the sentence would sound unnatural in normal Spanish. English often leaves out articles in a general or label-like way, but Spanish usually does not.

Because mochila is a feminine singular noun.

That means it takes:

• the article la
• feminine adjectives if any are added

For example:

• la mochila roja = the red backpack

The -a ending often signals feminine gender, though not always. In this case, mochila is feminine, so la mochila is correct.

Lleva is the third-person singular form of llevar in the present tense.

It matches the subject:

• El portaequipajes = singular
• therefore lleva = singular verb form

Compare:

• El portaequipajes lleva la mochila.
• Los portaequipajes llevan la mochila.

No subject pronoun is needed because Spanish normally leaves it out when the verb form already makes the subject clear.

Here llevar means something like:

• to carry
• to hold
• to bear
• to transport

Spanish llevar is a very broad verb. In other contexts it can also mean:

• to wear clothing
• to take something somewhere
• to bring along
• to lead

So the exact English translation depends on context. In this sentence, since the subject is el portaequipajes, the idea is that the luggage rack carries/holds the backpack.

Because llevar emphasizes the idea of carrying/supporting/transporting, while tener usually just means to have.

Compare:

• El portaequipajes lleva la mochila.
  = The luggage rack is carrying/holding the backpack.

• El portaequipajes tiene una mochila.
  = The luggage rack has a backpack.

The second one is grammatical, but it sounds less natural if you want to describe the backpack as being loaded onto the rack.

Word order matters because it changes the subject and object.

• El portaequipajes lleva la mochila.
  = The luggage rack carries the backpack.

• La mochila lleva el portaequipajes.
  = The backpack carries the luggage rack.

That second sentence is grammatically possible, but it means something different and probably strange.

Spanish word order is somewhat flexible, but the normal neutral order here is:

subject + verb + direct object

So El portaequipajes lleva la mochila is the most straightforward version.

Yes, but that would change the meaning.

• la mochila = the backpack, a specific backpack
• una mochila = a backpack, any backpack / an unspecified backpack

So:

• El portaequipajes lleva la mochila.
  refers to a specific backpack

• El portaequipajes lleva una mochila.
  means it is carrying some backpack, not necessarily one already known

You normally would not omit the article entirely in standard Spanish.

You could say:

Los portaequipajes llevan las mochilas.

Notice:

• el → los
• lleva → llevan
• la → las
• portaequipajes stays portaequipajes

That last point is important: the noun often looks the same in singular and plural, and the article tells you which one it is.

Because it is the thing being carried.

In the sentence:

• El portaequipajes = subject, the thing doing the action
• lleva = verb
• la mochila = direct object, the thing receiving the action

A useful test is to replace the direct object with a pronoun:

• El portaequipajes la lleva.
  = The luggage rack carries it.

Here la refers to la mochila, confirming that la mochila is the direct object.

Yes, it is widely understood, and it is a normal word across much of the Spanish-speaking world. Still, the exact object it refers to can vary a bit by context:

• a luggage rack
• a roof rack
• a baggage carrier
• sometimes a rack on a bicycle or vehicle

As with many nouns, regional preferences can differ, but portaequipajes is standard and broadly useful.

The stress falls on pa:

por-ta-e-qui-PA-jes

Since it ends in -s, Spanish spelling rules would normally stress the second-to-last syllable, and that is exactly what happens here.

Learners sometimes want to rush through the middle, but it helps to hear it in parts:

• porta
• equipajes

Then say it smoothly as one word: portaequipajes.