Fes servir l'arnès, a menys que sigui al transportí.

Fer servir is a very common Catalan expression meaning to use. Literally, it is something like make serve, but you should learn it as a fixed expression.

So:

• fes servir l'arnès = use the harness

Catalan also has verbs like utilitzar, but fer servir is extremely natural in everyday language.

Fes is the informal singular imperative of fer (to do / to make).

So the sentence is speaking to one person in an informal way:

• (tu) fes servir... = use...

Some related forms are:

• tu: fes servir
• vostè: faci servir
• vosaltres: feu servir
• vostès: facin servir

It is irregular, so it is not formed the way a regular verb would be.

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

So:

• Fes servir l'arnès
  really means
• (Tu) fes servir l'arnès

This is very normal in Catalan and much more common than always stating the pronoun.

Because the masculine singular article el becomes l' before a vowel sound.

So:

• el arnès → l'arnès

Also, the correct Catalan word is arnès, with è, not Spanish-style arnés.

So:

• l'arnès = the harness

A menys que means unless.

It introduces an exception:

• Fes servir l'arnès, a menys que...
  = Use the harness, unless...

It is a very useful expression, and one important thing about it is that it normally takes the subjunctive in the following clause.

Because a menys que normally requires the subjunctive.

So after a menys que, Catalan uses:

• sigui = present subjunctive of ser

Not:

• és = present indicative

That is why the sentence has:

• a menys que sigui...

and not

• a menys que és...

This is a subtle point. In Catalan, ser is often used for simple location in cases where English speakers might expect estar.

So:

• sigui al transportí is natural and standard

Using estigui is not impossible in every context, but it can sound more like you are emphasizing the state or placement. Here, sigui is the more neutral choice.

So for learners, the safest takeaway is:

• after a menys que, you need the subjunctive
• in this sentence, ser is the expected verb

Al is the contraction of:

• a + el = al

So:

• al transportí = in / to the carrier

This is very common in Catalan:

• al cotxe
• al parc
• al transportí

Transportí is a pet carrier or travel carrier, especially the kind used for animals.

So in this sentence it refers to the carrier the animal is placed in for transport.

It is a masculine noun:

• el transportí
• al transportí

The subject is understood from context and is not stated explicitly.

In a sentence like this, it usually refers to the animal, not the harness:

• a menys que [l'animal] sigui al transportí

Catalan often omits subjects when they are clear from the situation. English does this less freely, so this can feel strange at first.

If you wanted to make it fully explicit, you could say:

• Fes servir l'arnès, a menys que l'animal sigui al transportí.

Yes. Utilitza l'arnès is grammatical and natural.

But there is a slight difference in feel:

• fes servir l'arnès = very everyday, idiomatic
• utilitza l'arnès = also correct, sometimes a little more direct or slightly more formal depending on context

Both work, but fer servir is especially common in spoken Catalan.

They tell you about stress, and in the case of è, also about vowel quality.

• arnès: the stress is on the last syllable, and è is an open e
• transportí: the stress is on the final -tí

So the words are pronounced approximately like:

• ar-NÈS
• trans-por-ta-TÍ

For learners, the main thing is: do not ignore those accents, because they help you pronounce the words correctly.