Op het station koopt Sofie een broodje kaas.

This is because Dutch main clauses follow the V2 rule: the finite verb must be in the second position.

• If the sentence starts with the subject, you get: Sofie koopt een broodje kaas op het station.
• If the sentence starts with another element, like Op het station, the verb still stays second: Op het station koopt Sofie een broodje kaas.

So Op het station takes the first position, koopt must come second, and Sofie comes after the verb.

Usually op het station means at the station.

In Dutch, op is often used with places that English might express with at, especially for public places or locations in a general sense.

So:

• op het station = at the station
• not necessarily literally on top of the station

Dutch and English do not always use the same preposition for place expressions, so this is something you often just have to learn as a fixed phrase.

Because station is a het-word in Dutch.

Dutch nouns take either:

• de
• or het

There is no simple rule that always tells you which one to use, so you often have to learn the article together with the noun:

• het station
• de trein
• de winkel

A good habit is to memorize nouns with their article from the start.

Both are possible, but they are not exactly the same.

• een broodje kaas is the normal Dutch way to say a cheese sandwich / a cheese roll
• een broodje met kaas means more literally a roll with cheese

In Dutch, food combinations are often made with two nouns together:

• broodje kaas
• broodje ham
• broodje kroket

So broodje kaas is a very natural phrase.

Broodje is the diminutive form of brood.

• brood = bread
• broodje = little bread / bread roll / sandwich

In many everyday situations, broodje is used for a roll or a sandwich, depending on context.

The ending -je is a very common Dutch diminutive ending.

Because the subject is Sofie, which is third person singular.

The infinitive is kopen = to buy.
For hij/zij/het and singular names like Sofie, Dutch usually uses the stem + -t in the present tense:

• ik koop
• jij koopt
• hij/zij koopt
• Sofie koopt

So Sofie koopt = Sofie buys.

Yes, that is also correct.

Dutch word order is flexible in some parts of the sentence. These are all natural:

• Sofie koopt een broodje kaas op het station.
• Sofie koopt op het station een broodje kaas.
• Op het station koopt Sofie een broodje kaas.

The version in your sentence gives a bit more emphasis to where the action happens: at the station.

Because kaas is being used as part of the fixed expression broodje kaas.

It works a bit like a compound-style food label:

• a cheese sandwich
• a ham sandwich

You do not need a separate article before kaas here. The main noun phrase is:

• een broodje kaas

where broodje is the main noun, and kaas tells you what kind of sandwich it is.

Yes, it is in the present tense:

• koopt = buys / is buying

Like English, Dutch present tense can sometimes refer to the future if the context makes that clear. For example:

• Morgen koopt Sofie op het station een broodje kaas. = Tomorrow Sofie will buy a cheese sandwich at the station.

But without extra context, your sentence is normally understood as a present-time statement.

A very neutral version would be:

Sofie koopt op het station een broodje kaas.

or

Sofie koopt een broodje kaas op het station.

Starting with Op het station is perfectly correct, but it gives that location extra attention. Learners often first meet Dutch with subject-first sentences, so Sofie koopt ... may feel more familiar.

Yes, that is a very good idea.

It is useful to learn common location phrases as whole chunks, for example:

• op het station = at the station
• op school = at school
• op kantoor = at the office
• in de trein = on the train / in the train

Prepositions often do not match English exactly, so memorizing the whole phrase is easier than translating word by word.

A rough English-friendly guide is:

BROHT-yuh or BROHD-yuh

A few notes:

• oo is a long vowel, roughly like the vowel in go but without too much glide
• dj in broodje sounds a bit like the j in jar
• the final -je is a very common diminutive ending

The exact pronunciation depends a bit on accent, but recognizing -je as the diminutive ending is the main thing.