Au supermarché, je prends un panier parce que je n'achète que quelques légumes.

Au supermarché means at the supermarket here.

Au is the contraction of à + le:

• à + le = au
• à + les = aux

So:

• au supermarché = at/to the supermarket
• aux États-Unis = in/to the United States

French normally uses this contraction before a masculine singular noun with le.

The phrase Au supermarché is an introductory location phrase, setting the scene first: At the supermarket, ...

The comma helps separate that introductory phrase from the main clause:

• Au supermarché, je prends...

In short:

• with the comma, it sounds natural and clear
• without it, the sentence would still usually be understood, but the comma is very common when a place or time phrase comes first

Yes. The French present tense often covers both:

• I take / I buy
• I am taking / I am buying

So:

• je prends can mean I take or I’m taking
• j’achète can mean I buy or I’m buying

French does not need a separate tense like English am taking in many everyday situations. Context tells you whether it is a general habit or something happening right now.

Yes. Prendre is an irregular verb.

Its present-tense forms begin:

• je prends
• tu prends
• il/elle prend
• nous prenons
• vous prenez
• ils/elles prennent

A few useful things to notice:

• the s in prends is part of the verb form
• the final s is normally not pronounced
• the verb stem changes a bit across the conjugation: prend-, pren-, prenn-

So you simply have to learn je prends as the correct form.

Here prendre means to take or to pick up, not to buy.

So:

• je prends un panier = I take/pick up a basket
• j’achète un panier = I buy a basket

In a supermarket, you usually take a basket to carry your shopping. You are not buying the basket itself.

Panier is a masculine noun, so it takes un in the singular:

• un panier

If it were feminine, it would take une.

Also, un is used because this is one basket / a basket, not a specific previously mentioned basket.

A useful vocabulary note:

• un panier = a basket
• un chariot or sometimes un caddie = a shopping cart/trolley

Parce que means because and introduces the reason:

• je prends un panier parce que... = I take a basket because...

It connects the action and the explanation:

• action: je prends un panier
• reason: je n’achète que quelques légumes

This is the most common everyday way to say because in French.

This is the very important structure ne ... que, which means only.

So:

• je n’achète que quelques légumes literally works like:
• I do not buy except a few vegetables but the natural English meaning is:
• I only buy a few vegetables

A key point: ne ... que is not a normal negative like ne ... pas. It limits something instead.

Examples:

• Je ne bois que de l’eau. = I only drink water.
• Il ne travaille que le matin. = He only works in the morning.

In your sentence, que comes right before quelques légumes, so that is the part being limited.

This is called elision.

When ne comes before a word beginning with a vowel sound, the e drops and an apostrophe is used:

• ne achète becomes n’achète

The same thing happens with other words:

• je aime becomes j’aime
• le ami becomes l’ami
• si il becomes s’il

So je n’achète is just the normal shortened form before a vowel.

Quelques means a few or some.

In this sentence, it suggests a small number:

• quelques légumes = a few vegetables

Compared with other options:

• des légumes = some vegetables, more neutral
• quelques légumes = a few vegetables, clearly a small amount
• plusieurs légumes = several vegetables

So quelques fits well with the idea that a basket is enough.

Because quelques already acts as the determiner.

In French, you usually do not put another article like des before it:

• quelques légumes = correct
• des quelques légumes = not correct here

This is similar to English:

• a few vegetables not
• some a few vegetables

So quelques already does the job of introducing the noun.

Yes. French can also say:

• Je prends un panier au supermarché parce que je n’achète que quelques légumes.

That is also grammatical.

But starting with Au supermarché puts the location first, as a kind of scene-setting phrase. It is very natural, especially in textbook examples and everyday speech when you want to frame the situation first.