La corretja i l'arnès són diferents.

Because Catalan usually shortens el and la to l' before a word that begins with a vowel or silent h.

• el arnès → l'arnès
• la escola → l'escola

So arnès is a masculine noun, but before a-, the article becomes l'.

The article depends on both gender and the first sound of the noun.

• corretja is feminine, so it takes la
• arnès is masculine, so it takes el, but before a vowel that becomes l'

So:

• la corretja
• el arnès → l'arnès

A good habit is to learn nouns together with their article: la corretja, l'arnès.

Catalan normally repeats the definite article with each specific noun in a list or pair:

• La corretja i l'arnès

This sounds natural and standard. English often does the same, but Catalan is especially comfortable repeating the article when talking about two distinct things.

Because the subject is plural.

The subject is:

• La corretja i l'arnès

That means the strap and the harness — two things joined by i (and). Two things make a plural subject, so the verb must be plural too:

• singular: La corretja és diferent
• plural: La corretja i l'arnès són diferents

són is the 3rd person plural of ser.

Because the adjective must agree in number with the subject.

Since la corretja i l'arnès is plural, the adjective is plural too:

• singular: diferent
• plural: diferents

A useful detail: diferent has the same singular form for masculine and feminine. Only the plural changes here.

Catalan uses ser because the sentence is stating an inherent difference between two things, not a temporary state.

• són diferents = they are different
• this is about what they are like, not how they happen to be at the moment

So ser is the natural choice.

The accents show stress, and in Catalan they also help show vowel quality.

• arnès has è: the stress falls on the last syllable, and the e is an open e
• són has ó: the word is stressed, and the o is a closed o

These accents are part of the correct spelling and should not be omitted.

A rough pronunciation guide is:

• corretja ≈ kuh-REJ-uh
• arnès ≈ ar-NESS

A few useful pronunciation notes:

• the r in Catalan is usually a quick tap
• tj in corretja sounds like the j in judge
• ès in arnès has a clearly stressed final syllable

Pronunciation varies somewhat by region, but those approximations are good enough for a learner.

Not always. Corretja can mean different things depending on context, such as:

• strap
• leash
• belt
• band

So the exact English translation depends on what kind of object you are talking about. Arnès is more specifically harness.

That is why this sentence can be useful: it clearly contrasts two different objects.

Yes. i means and.

So:

• La corretja i l'arnès = The strap and the harness

A small useful point: in Catalan, i stays i before vowels, so i l'arnès is completely normal.