Můj syn nechce šunku v sendviči, raději má sýr a okurku.

Because můj has to agree with the noun it describes.

• syn = son
• syn is masculine animate singular
• so the correct form of my is můj

Compare:

• můj syn = my son
• moje dcera = my daughter
• moje dítě = my child

So můj is the masculine singular nominative form.

Nechce is the 3rd person singular form of chtít (to want).

The subject is můj syn = my son, so Czech uses the form meaning he does not want:

• chci = I want
• chceš = you want
• chce = he/she/it wants

With negation:

• nechci = I do not want
• nechceš = you do not want
• nechce = he/she/it does not want

So můj syn nechce... = my son doesn’t want...

Because it is the direct object of nechce.

The basic form is:

• šunka = ham

But after a transitive verb like chtít/nechtít, the thing wanted or not wanted usually goes into the accusative case:

• šunka → šunku

So:

• Můj syn nechce šunku. = My son doesn’t want ham.

This is a very common pattern in Czech:

• mám kávu = I have coffee
• vidím dívku = I see a girl
• chci vodu = I want water

Because the preposition v (in) usually takes the locative case when it means location.

• sendvič = sandwich
• locative singular: v sendviči = in the sandwich

So:

• v domě = in the house
• v autě = in the car
• v sendviči = in the sandwich

This is one of the key things to learn in Czech: prepositions often require a specific case.

Here it means in the sandwich in the food sense: as a filling or ingredient.

So šunku v sendviči means ham in a sandwich / as part of the sandwich.

English sometimes says in a sandwich, sometimes just on a sandwich or in his sandwich, depending on the food. Czech v sendviči is the normal way to express that the ingredient is part of the sandwich.

Raději means rather, preferably, or instead.

In this sentence it shows preference:

• raději má sýr a okurku = he prefers cheese and cucumber instead

It often appears in sentences like:

• Dám si raději čaj. = I’d rather have tea.
• Půjdu raději pěšky. = I’d rather go on foot.

So here it contrasts with šunku:

• he does not want ham;
• instead / rather, he prefers cheese and cucumber.

Yes, here raději má expresses preference, similar to prefers.

Czech often uses the verb mít rád (to like) and its comparative idea mít raději (to like better / prefer).

So:

• má rád = likes
• má raději = likes better / prefers

In this sentence, the meaning is:

• raději má sýr a okurku = he prefers cheese and cucumber

A learner may also see:

• Můj syn má raději sýr a okurku než šunku. = My son prefers cheese and cucumber to ham.

That is a slightly fuller version.

Both words are direct objects here, but different noun types can have different accusative forms.

• sýr is a masculine inanimate noun
• in the singular, masculine inanimate nouns often have the same form in nominative and accusative
• so sýr stays sýr

But:

• okurka is a feminine noun
• feminine nouns like this usually change in the accusative
• okurka → okurku

So:

• má sýr = has cheese
• má okurku = has cucumber

Because Czech, like English, often leaves out repeated information when it is already clear.

The full idea is:

• He doesn’t want ham in the sandwich;
• he prefers cheese and cucumber in the sandwich.

But saying v sendviči only once is enough.

A more explicit version could be:

• Můj syn nechce v sendviči šunku, raději má v sendviči sýr a okurku.

That is grammatically possible, but less natural because it repeats too much.

Yes, Czech word order is flexible, and another very natural version would be:

• Můj syn nechce v sendviči šunku, raději má sýr a okurku.

Both are understandable. The difference is mostly about emphasis.

• nechce šunku v sendviči can sound like emphasis on ham in the sandwich
• nechce v sendviči šunku can sound a bit more neutral or natural in everyday Czech

Czech word order often shifts according to focus and emphasis, much more than English.

Yes. Sendvič is a normal Czech word for sandwich.

Its declension here is:

• nominative: sendvič
• locative after v: sendviči

So v sendviči is exactly what you would expect from the noun in that case.

Yes, but the meaning would change slightly.

• nechce šunku = he does not want ham
• nemá rád šunku = he does not like ham

So:

• nechce is about what he wants in this situation
• nemá rád is about general taste or preference

If the idea is specifically about this sandwich choice, nechce is a good fit.
If the idea is that he generally dislikes ham, then nemá rád šunku would be more appropriate.

Because the sentence has two coordinated parts:

• Můj syn nechce šunku v sendviči
• raději má sýr a okurku

In English, you might also separate such clauses with a comma:

• My son doesn’t want ham in his sandwich, he’d rather have cheese and cucumber.

Czech punctuation often uses a comma to separate these kinds of linked clauses, especially when the second part contrasts with the first.

It means cheese and cucumber — two items, taken together.

So the son prefers a sandwich with:

• sýr = cheese
• okurku = cucumber

Czech does not need any article here, just like many food descriptions in English:

• with cheese and cucumber
• sýr a okurku

The sentence naturally suggests these are the preferred fillings instead of ham.