No sabes lo bien que sabe un bocadillo caliente después de un viaje largo.

Because No sabes... is an idiomatic way to say You have no idea... in Spanish.

It is not really meant as a literal statement that the other person lacks knowledge. It is a very common expressive pattern:

• No sabes lo difícil que fue. = You have no idea how hard it was.
• No sabes lo feliz que estoy. = You have no idea how happy I am.

So here, No sabes lo bien que sabe... means something like:

• You have no idea how good ... tastes
• You don’t know how good ... tastes

Yes. They are both forms of saber, but with two different meanings.

1. saber = to know

   • No sabes = You don’t know / You have no idea

2. saber = to taste

   • sabe = it tastes

So in this sentence:

• No sabes = You have no idea
• lo bien que sabe un bocadillo caliente = how good a hot sandwich tastes

This dual meaning is very common in Spanish, so learners often notice it right away.

Lo bien que... is a very common structure used to emphasize degree, especially after verbs like saber, estar, sentirse, etc.

So:

• No sabes lo bien que sabe... = You have no idea how good it tastes
• No imaginas lo cansado que estoy. = You can’t imagine how tired I am

Here, lo does not mean it or the in the usual sense. It is part of an exclamatory/emphatic structure:

• lo + adjective/adverb + que

Examples:

• lo difícil que es = how difficult it is
• lo rápido que corre = how fast he runs
• lo bien que sabe = how good it tastes

You may also hear qué bien sabe, but in this sentence No sabes lo bien que sabe... is the more natural fixed pattern.

Because bien is an adverb, and it modifies the verb sabe.

• sabe bien = it tastes good / it tastes nice

In English, we say good, but Spanish uses the adverb bien with this verb.

Compare:

• El bocadillo está bueno. = The sandwich is tasty / good
• El bocadillo sabe bien. = The sandwich tastes good

So in this sentence, since the idea is about how it tastes, bien is the normal choice.

Here sabe means tastes.

• Un bocadillo caliente sabe bien. = A hot sandwich tastes good.

This is similar to está bueno, but not exactly identical.

• sabe bien focuses on the taste
• está bueno describes the food as tasty / good to eat

Both can work in food contexts, but saber is specifically about flavor or taste.

For example:

• La sopa sabe muy bien. = The soup tastes very good.
• La sopa está muy buena. = The soup is very tasty.

Because Spanish often omits subject pronouns when the verb already makes the subject clear.

• No sabes already tells you the subject is you (informal singular, tú).
• Spanish does not need to say tú unless it wants emphasis or contrast.

So:

• No sabes lo bien que sabe... = normal, natural
• Tú no sabes lo bien que sabe... = more emphatic, like You have no idea...

The subject of sabe is un bocadillo caliente.

So the structure is:

• No sabes = main clause
• lo bien que sabe un bocadillo caliente... = subordinate clause

Literally, the second part is something like:

• how well a hot sandwich tastes

In more natural English:

• how good a hot sandwich tastes

So un bocadillo caliente is the thing that tastes good.

Not exactly.

In Spain:

• bocadillo usually means a sandwich made with Spanish bread, often a baguette-style roll or loaf
• sándwich usually means a sandwich made with sliced bread

So a bocadillo caliente would typically be a hot sandwich in a crusty bread roll, not a sliced-bread sandwich.

This is a useful cultural vocabulary point for Spanish from Spain.

Because in Spanish, descriptive adjectives usually come after the noun.

So:

• un bocadillo caliente = a hot sandwich
• un viaje largo = a long journey

This is the normal order in Spanish.

Sometimes adjectives can go before the noun for stylistic or meaning-related reasons, but here the post-noun position is the standard and most natural one.

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

• un viaje largo is the neutral, most straightforward way to say a long journey/trip
• un largo viaje sounds a bit more literary, expressive, or stylistic

In everyday Spanish, un viaje largo is the more natural choice here.

Después de means after.

It is a fixed prepositional expression:

• después de + noun
• después de + infinitive

In this sentence:

• después de un viaje largo = after a long journey

Other examples:

• después de la cena = after dinner
• después de trabajar = after working

So here it tells you when that hot sandwich tastes especially good.

Yes. That is a very natural English translation.

A more literal version would be:

• You don’t know how well a hot sandwich tastes after a long journey

But natural English prefers:

• You have no idea how good a hot sandwich tastes after a long journey

So this is a good example of a sentence where the most natural translation is not the most word-for-word one.