Poso el collar i l'arnès a la mascota.

Poso comes from the verb posar, which means to put, to place, or in this kind of context, to put on.

So Poso el collar i l'arnès a la mascota means that I put / I’m putting the collar and the harness on the pet.

Catalan often leaves out subject pronouns when they are clear from the verb ending.

Here, poso already tells you the subject is I, so jo is unnecessary. You could say Jo poso el collar i l'arnès a la mascota, but it would usually add emphasis, as in I’m the one putting them on.

Because the masculine singular article el becomes l' before a word that starts with a vowel or silent h.

So:

• el collar
• l'arnès

This is just the normal apostrophe rule in Catalan.

In Catalan, when you say that you are putting something on a person or animal, it is very common to use a before that person or animal.

So posar alguna cosa a algú means to put something on someone.

Here:

• el collar i l'arnès = the things being put on
• a la mascota = the animal receiving them

English does not always use a preposition in the same way, so this may feel different at first.

Because mascota is a feminine noun.

In Catalan:

• a + el = al
• a + la = a la (no contraction)

So:

• al gos = to/on the dog
• a la mascota = to/on the pet

Yes. Mascota is grammatically feminine, regardless of whether the animal itself is male or female.

So you say:

• la mascota

even if the pet is a male dog, for example.

This is grammatical gender, not biological sex.

Yes. That is very natural Catalan.

Li means to him / to her / to it, and here it refers to la mascota. So:

• Poso el collar i l'arnès a la mascota
• Li poso el collar i l'arnès a la mascota

Both are understandable, but the version with li is often especially natural when the receiver is a person or animal.

Because each noun normally keeps its own article.

So Catalan says:

• el collar i l'arnès

not normally:

• el collar i arnés

This is similar to saying the collar and the harness in English. Each item is definite, so each one gets its article.

• collar = collar
• arnès = harness

So the sentence is talking about two different items: the collar and the harness.

Arnès is stressed on the last syllable: ar-NÈS.

The grave accent in è shows that the vowel is an open e sound. It also helps mark the stress pattern.

When it appears with the article, l'arnès, you pronounce it smoothly together, without a pause.

It can mean either, depending on context.

Catalan present tense often covers both:

• I put
• I’m putting

If you want to make the ongoing action very explicit, Catalan can also use:

• Estic posant el collar i l'arnès a la mascota = I’m putting the collar and the harness on the pet

So poso is perfectly normal here.