El supermercat on vaig avui és al costat de la fleca.

On means where here.

It introduces a relative clause that describes el supermercat:

• el supermercat = the supermarket
• on vaig avui = where I’m going today

So on connects the place noun to the clause about it, just like where in English.

This is a very common point of confusion.

Vaig is the 1st person singular present tense of anar (to go):

• jo vaig = I go / I am going

But vaig also appears in the periphrastic past:

• vaig anar = I went

So the key difference is:

• vaig by itself here = I go / I’m going
• vaig + infinitive = usually a past meaning, such as vaig anar = I went

In your sentence, there is no infinitive after vaig, so it is simply the present tense of anar.

Catalan often uses the present tense for something happening today, especially if it is planned, current, or very near in time.

So on vaig avui can naturally mean:

• where I’m going today

This is similar to English using the present progressive for a near-future plan:

• I’m going to the supermarket today

If you want to make the future idea more explicit, Catalan could also use:

• El supermercat on aniré avui... = The supermarket I’ll go to today...

But the present tense is very normal here.

Avui is an adverb, and Catalan adverb placement is somewhat flexible, but on vaig avui sounds natural and straightforward.

The structure is:

• on = where
• vaig = I go / I’m going
• avui = today

So literally it is something like:

• where I’m going today

You could move avui in some sentences for emphasis, but in this sentence its position is very normal.

És al costat de means is next to or is beside.

It is a fixed expression:

• al costat de = next to / beside

For example:

• És al costat de la fleca = It is next to the bakery

Also note that al is a contraction:

• a + el = al

So al costat literally comes from a el costat, though you always say al costat.

Because al costat is normally followed by de.

The full pattern is:

• al costat de + noun

Examples:

• al costat de la fleca = next to the bakery
• al costat del banc = next to the bank
• al costat de casa = next to the house/home

So the de is required by the expression.

Fleca means bakery or baker’s shop.

It is a correct and common Catalan word. Depending on region and context, you may also hear:

• forn
• forn de pa

These are also often used for a bakery or bread shop.

So:

• la fleca = the bakery
• el forn / el forn de pa = the bakery / bread shop

All are useful to know.

Catalan uses definite articles very regularly with nouns, including shops and places.

So:

• el supermercat = the supermarket
• la fleca = the bakery

That is completely normal. In this sentence, the article works much like English the.

It can technically cover both kinds of present meaning, because Catalan present tense is broad.

Depending on context, vaig can mean:

• I go
• I am going

In this sentence, because of avui, the most natural interpretation is:

• where I’m going today

If it were a habit, you would usually have a context like:

• on vaig cada dissabte = where I go every Saturday

So avui pushes the meaning toward a current or near-future event, not a habit.

Yes.

A more explicitly future version would be:

• El supermercat on aniré avui és al costat de la fleca.

Here:

• aniré = I will go

That said, the original sentence with vaig is still natural, because Catalan often uses the present for near-future plans.

For a place in a relative clause like this, on is the normal choice.

Here it works very naturally because supermercat is a place:

• El supermercat on vaig avui...

A learner should mainly remember:

• on is the usual word for where
• it is especially common after nouns referring to places

So in this sentence, on is exactly what you would expect.