Mému synovi chutná sendvič se sýrem a okurkou.

Because the verb chutnat works like to taste good to someone, not like the English verb to like.

So in Czech, the person who experiences the taste is put in the dative:

• Mému synovi chutná... = My son likes / enjoys the taste of...
• literally: To my son, ... tastes good

This is similar to other Czech verbs where the experiencer is in the dative, such as:

• Líbí se mi to. = I like it.
• literally: It is pleasing to me

So mému synovi means to my son.

Because můj syn is the nominative form, used for the subject of a sentence, while here my son is not the grammatical subject.

Compare:

• Můj syn jí sendvič. = My son is eating a sandwich.
  Here můj syn is the subject.

• Mému synovi chutná sendvič. = My son likes the sandwich / The sandwich tastes good to my son.
  Here sendvič is the grammatical subject, and mému synovi is in the dative.

Also, both words change:

• můj → mému
• syn → synovi

because both must match the dative singular.

Because the grammatical subject is sendvič, which is singular.

In Czech, the verb agrees with the thing that tastes good, not with the person who likes it.

So:

• Mému synovi chutná sendvič.
  sendvič = singular → chutná

If the thing were plural, the verb would also be plural:

• Mému synovi chutnají sendviče. = My son likes sandwiches.

This is often surprising for English speakers because English normally makes my son feel like the subject, but Czech grammar treats sendvič as the subject here.

Yes, grammatically it is.

With chutnat, the thing that tastes good is in the nominative, and the person receiving the sensation is in the dative.

So in:

• Mému synovi chutná sendvič se sýrem a okurkou

the roles are:

• mému synovi = dative, the experiencer
• sendvič = nominative, the subject
• se sýrem a okurkou = a phrase describing the sandwich

A useful literal pattern is:

• [To someone-DATIVE] tastes good [thing-NOMINATIVE].

Not exactly.

Chutnat is specifically about taste or the enjoyment of food and drink.

• Mému synovi chutná sendvič. = My son likes the sandwich / The sandwich tastes good to my son.

But mít rád is broader and means to like in a more general sense:

• Můj syn má rád sendviče. = My son likes sandwiches.

A rough difference:

• chutnat = to taste good, to be tasty to someone
• mít rád = to like, to be fond of

For food, both can sometimes be possible, but chutnat focuses more on the taste experience.

The preposition s / se here means with, and it requires the instrumental case.

So:

• sýr → sýrem
• okurka → okurkou

That is why you get:

• se sýrem a okurkou = with cheese and cucumber

As for s vs se:

• s is the basic form
• se is used for easier pronunciation before some consonant combinations

Since s sýrem would be awkward to say, Czech uses:

• se sýrem

This extra e is just a pronunciation aid; the meaning is the same.

Because the preposition se applies to both nouns:

• se sýrem a okurkou = with cheese and cucumber

In Czech, when two nouns are joined by a (and) under the same preposition, both normally take the case required by that preposition.

So both are in the instrumental:

• sýrem
• okurkou

You do not need to repeat se before the second noun.

Mému is the dative singular masculine/neuter form of můj (my).

Here it agrees with synovi, which is masculine singular dative.

Basic forms of můj include:

• můj syn = my son nominative
• mého syna = genitive/accusative
• mému synovi = dative

So mému means to my in this sentence.

Learners often notice that possessives decline just like adjectives, and that is exactly what is happening here.

In standard Czech, synovi is the normal dative singular form.

For some masculine nouns, Czech sometimes has two dative singular endings, but with syn, synovi is the usual correct form.

So you should learn:

• můj syn
• bez syna
• k synovi
• se synem

For practical purposes, use mému synovi.

Yes. Czech word order is flexible because the cases show the grammatical roles.

So these can all be possible depending on emphasis:

• Mému synovi chutná sendvič se sýrem a okurkou.
• Sendvič se sýrem a okurkou mému synovi chutná.
• Sendvič mému synovi chutná.

However, the original order sounds natural if you are introducing the person first and then saying what tastes good to him.

Word order in Czech often reflects topic and focus rather than basic grammatical function.

You would make the subject plural, and the verb would change to agree with it:

• Mému synovi chutnají sendviče se sýrem a okurkou.

Changes:

• sendvič → sendviče
• chutná → chutnají

The dative phrase stays the same:

• mému synovi

So the pattern is:

• singular: Mému synovi chutná sendvič.
• plural: Mému synovi chutnají sendviče.

Czech has no articles like a and the, so the sentence by itself does not explicitly say which one is meant.

• sendvič can mean a sandwich or the sandwich
• context tells you which meaning is intended

So depending on context, this could mean:

• My son likes a sandwich with cheese and cucumber
• My son likes the sandwich with cheese and cucumber
• A sandwich with cheese and cucumber tastes good to my son

Czech often leaves this kind of definiteness unstated.

It is pronounced approximately KHOOT-naa, with ch like the sound in the Scottish loch or German Bach.

In this sentence, chutná literally means tastes good or is tasty.

So:

• Sendvič mému synovi chutná
  literally: The sandwich tastes good to my son

But in natural English, this is often translated as:

• My son likes the sandwich

So the Czech verb itself is closer to taste good than to the general English verb like.