El cogombre és a la nevera, al costat de l'enciam que vam comprar ahir al mercat.

In standard Catalan, ser is very commonly used to state the simple location of something, especially when you are just answering where is it?

So El cogombre és a la nevera is a very normal way to say The cucumber is in the fridge.

You may also hear estar in some varieties or in more colloquial speech, especially if the speaker wants to stress the temporary position or state, but és is completely natural here.

Al costat de means next to, beside, or more literally at the side of.

It is a fixed expression:

• al costat de la porta = next to the door
• al costat del llibre = next to the book

So al costat de l'enciam means next to the lettuce.

Because al is the contraction of a + el.

So:

• a + el costat → al costat
• a + el mercat → al mercat

But nevera is feminine, so it takes la, not el:

• a + la nevera → a la nevera

Catalan contracts a + el to al, but not a + la.

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

So:

• el enciam → l'enciam
• el amic → l'amic
• el home → l'home

This is just the normal apostrophe rule in Catalan.

This is for the same reason: before a vowel, el changes to l'.

So:

• de + el mercat → del mercat
• but de + l'enciam stays de l'enciam

You only get del when the article is fully el. If the noun starts with a vowel and the article becomes l', then you write de l'...

Here que is a relative pronoun, meaning that or which.

It introduces extra information about l'enciam:

l'enciam que vam comprar ahir al mercat

= the lettuce that we bought yesterday at the market

So que links l'enciam to the clause that describes it.

Because Catalan very often uses the periphrastic past, which is formed with the present tense of anar plus an infinitive.

So:

• vaig comprar = I bought
• vas comprar = you bought
• va comprar = he/she bought
• vam comprar = we bought

Even though vam comes from anar, this structure does not mean we go buy. It is simply a normal past tense in Catalan.

Catalan often leaves out subject pronouns because the verb ending already shows who the subject is.

In vam comprar, the form vam already tells you it means we.

So:

• vam comprar = we bought
• nosaltres vam comprar = we bought, with extra emphasis

The pronoun nosaltres is optional unless you want contrast or emphasis.

It describes the lettuce.

The relative clause comes right after l'enciam, so the most natural reading is:

the lettuce that we bought yesterday at the market

It does not mean that the cucumber was bought yesterday at the market.

Yes, it could move. Catalan word order is fairly flexible.

The version in the sentence, que vam comprar ahir al mercat, is natural and neutral.

You could also say:

• que ahir vam comprar al mercat

That puts a little more focus on ahir.

So the given order is not the only possible one, but it is a very common and natural choice.

Catalan uses definite articles very regularly with specific nouns.

In this sentence, the speaker is referring to particular, identifiable things:

• the cucumber
• the fridge
• the lettuce
• the market

So the definite articles are exactly what you would expect. Catalan article use is often a bit broader than English in some contexts, but here it matches English quite closely.

Yes. Enciam is masculine, so the singular article is el, which becomes l' before the vowel:

• l'enciam
• l'enciam fresc = the fresh lettuce

This is something you mostly just have to learn with the noun. In Catalan, noun gender does not always match the ending.