El desmuntable és al costat de la clau anglesa.

Because el is the masculine singular definite article, meaning the.

Here, desmuntable is being treated as a specific, identifiable object, so Catalan uses the definite article:

• el desmuntable = the ...
• la clau anglesa = the ...

Catalan uses definite articles very regularly, often in places where English also uses the.

In this sentence, it functions as a noun.

By itself, desmuntable is often an adjective meaning something like detachable, removable, or able to be taken apart, depending on context. But in tool vocabulary, words like this can also be used substantively: that is, an adjective is used as a noun because the object is understood from context.

So here:

• el desmuntable = a specific tool/object called desmuntable

This is not unusual in Catalan.

Because in Catalan, ser is very commonly used for simple location, especially in standard language.

So a sentence like this naturally uses:

• és al costat de ... = is next to ...

A native English speaker may expect a contrast like Spanish ser vs estar, but Catalan does not match Spanish exactly. In Catalan, ser is often the normal choice for saying where someone or something is.

So El desmuntable és al costat de la clau anglesa is perfectly natural.

Al costat de is a fixed expression meaning:

• next to
• beside
• literally, at the side of

It is built like this:

• costat = side
• al = at the / to the
• de = of

So literally, al costat de la clau anglesa means at the side of the adjustable wrench.

You should learn al costat de as a whole expression.

Because a + el contracts to al in Catalan.

So:

• a + el costat → al costat

This is a normal contraction, just like:

• de + el → del

So you say:

• al costat
• not a el costat

Because the expression al costat needs de before the thing it is next to.

So the structure is:

• al costat de + noun

Examples:

• al costat de la porta = next to the door
• al costat del cotxe = next to the car
• al costat de la clau anglesa = next to the adjustable wrench

So the de is required by the expression.

Because anglesa must agree with clau, and clau is a feminine noun.

In Catalan, adjectives usually agree in gender and number with the noun they describe:

• masculine singular: anglès
• feminine singular: anglesa

Since clau is feminine singular, the adjective must also be feminine singular:

• la clau anglesa

Even though the literal parts may look odd to an English speaker, this is simply the normal fixed name of the tool.

Yes, literally the words are:

• clau = key / wrench / spanner, depending on context
• anglesa = English

But as a whole, clau anglesa is the normal name of a specific tool. So you should understand it as a fixed expression, not by translating each word separately every time.

This is common in many languages: the literal meaning and the actual tool name are not always the same thing.

The accent shows that és is stressed and helps distinguish it from es, which is a different word.

• és = is
• es = a pronoun used in reflexive/passive structures

So the accent is important for spelling and meaning.

Examples:

• és aquí = is here
• es renta = washes himself / herself

Yes, but the original order is the most straightforward and neutral:

• El desmuntable és al costat de la clau anglesa.

Catalan often uses:

• subject + verb + place

You could change the order for emphasis, for example:

• Al costat de la clau anglesa hi és el desmuntable.

But that sounds more marked or stylistically different. For a learner, the original sentence is the best pattern to follow.

Yes, in this sentence you do.

Catalan normally keeps the definite article with both nouns when referring to specific objects:

• el desmuntable
• la clau anglesa

English sometimes uses fewer articles in certain expressions, but Catalan usually does not omit them here. So the full phrase is natural and correct exactly as written.