Im Supermarkt kaufe ich einen Kürbis, Sellerie und zwei Süßkartoffeln.

Because German main clauses follow the verb-second rule. The conjugated verb must come in the second position.

So if you put Im Supermarkt at the beginning, that whole phrase takes the first position, and the verb kaufe must come next:

Im Supermarkt | kaufe | ich ...

This is one of the biggest word-order differences from English.

Im is a contraction of in dem.

• in = in
• dem = the, masculine/neuter dative article

So:

• im Supermarkt = in dem Supermarkt

This is used here because the sentence describes a location, not motion into somewhere.

Because in can take either dative or accusative, depending on meaning:

• dative for location: where?
• accusative for direction/movement: where to?

Here, im Supermarkt means in the supermarket as a location, so German uses the dative.

Compare:

• Ich kaufe im Supermarkt. = I buy things in the supermarket.
• Ich gehe in den Supermarkt. = I go into the supermarket.

Because Kürbis is a masculine noun: der Kürbis.

It is also the direct object of kaufen, so it must be in the accusative case. For masculine nouns, the indefinite article changes like this:

• nominative: ein Kürbis
• accusative: einen Kürbis

So einen tells you that Kürbis is being bought.

German often leaves out the article with foods, ingredients, and shopping-list-style items when speaking generally.

So ich kaufe Sellerie is natural, just like saying you are buying celery as a food item.

If you wanted to emphasize a single whole item in some contexts, you might also hear einen Sellerie, especially if the meaning is closer to one celeriac/celery root. But in everyday shopping language, the version without an article is very common.

After numbers like zwei, German usually does not use an article unless you mean something very specific.

So:

• zwei Süßkartoffeln = two sweet potatoes

The number already shows quantity, so no article is needed.

Also, because the number is more than one, the noun must be plural:

• singular: die Süßkartoffel
• plural: die Süßkartoffeln

Yes. They are all part of what is being bought, so they are all direct objects of kaufe.

You can think of the sentence as:

• einen Kürbis
• Sellerie
• zwei Süßkartoffeln

All three belong to the same object list after kaufe.

The accusative is easiest to see in einen Kürbis, because the article changes visibly. With Sellerie and zwei Süßkartoffeln, the case is not marked as clearly on the noun phrase itself.

Here they are:

• der Supermarkt
• der Kürbis
• der Sellerie
• die Süßkartoffel

A dictionary usually gives the noun in the nominative singular together with its gender.

That is why you learn:

• der Kürbis, not einen Kürbis
• die Süßkartoffel, not Süßkartoffeln

Because it is the plural form of die Süßkartoffel.

• singular: die Süßkartoffel
• plural: die Süßkartoffeln

Also, Süßkartoffel is a compound noun:

• süß = sweet
• Kartoffel = potato

In German compounds, the last part determines the gender and basic plural pattern. Since the head noun is Kartoffel, the compound behaves like Kartoffel:

• die Kartoffel → die Kartoffeln
• die Süßkartoffel → die Süßkartoffeln

Yes, absolutely.

Both are correct:

• Im Supermarkt kaufe ich ...
• Ich kaufe im Supermarkt ...

The difference is mainly emphasis and information structure.

• Im Supermarkt kaufe ich ... puts the location first, as the setting or topic.
• Ich kaufe im Supermarkt ... is a more neutral subject-first sentence.

German is flexible in this way, but the verb still stays in second position.

Because in German, all nouns are capitalized, not just proper names.

So in this sentence you get:

• Supermarkt
• Kürbis
• Sellerie
• Süßkartoffeln

This is a standard rule of German spelling and helps readers spot nouns quickly.

Because this is a simple list.

German punctuation works like this here:

• commas separate earlier items in a series
• normally there is no comma before und in a simple list

So:

einen Kürbis, Sellerie und zwei Süßkartoffeln

is the normal punctuation.

Two common learner points:

• ü is an umlaut vowel. It does not sound like plain u.
• ß is pronounced like ss.

So:

• Kürbis has the ü sound
• Süßkartoffeln has both ü and ß

A rough guide:

• Kürbis ≈ start with a sound somewhat like saying ee with rounded lips
• Süß ≈ sounds roughly like süüs, with ß pronounced as ss

If you cannot type umlauts, German often allows replacements:

• ü → ue
• ß → ss

So you may see:

• Kuerbis
• Suesskartoffeln

These are typing substitutes, not the standard spellings when the proper characters are available.