hourensou to daikon no sarada ha, omotta yori oisikatta.

A very literal breakdown is:

• ほうれん草と大根のサラダ = spinach-and-daikon salad
• は = as for...
• 思ったより = than I thought / than expected
• おいしかった = was delicious / tasted good

So the structure is basically:

As for the spinach-and-daikon salad, it was more delicious than I thought.

Japanese often puts the topic first, and the comment about it comes afterward.

と is the particle used to connect nouns, often meaning and.

So:

• ほうれん草と大根 = spinach and daikon

In this sentence, it links the two ingredients of the salad.

Here, の connects nouns and shows a relationship between them. A very natural translation is:

• ほうれん草と大根のサラダ = a salad of spinach and daikon / a spinach-and-daikon salad

You can think of の here as something like:

• made of
• with
• consisting of

It is not always exactly of, but that is often a good starting point for learners.

は marks the topic of the sentence.

So ほうれん草と大根のサラダは means:

• As for the spinach-and-daikon salad...
• The spinach-and-daikon salad...

It tells you what the speaker is about to comment on. The comment is:

• 思ったよりおいしかった = was tastier than I thought

より means than in comparisons.

• 思った = thought
• より = than

So 思ったより literally means:

• than [I] thought

A fuller version could be something like:

• 思ったよりおいしかった = it was more delicious than I thought it would be

Japanese often leaves out words that are understood from context, so you do not need to say the full comparison explicitly.

Usually it is understood to be the speaker: than I thought.

Japanese often omits the subject when it is obvious from context. So:

• 思ったより usually means than I thought
• in some contexts, it could mean than someone thought, if that is already clear

In an ordinary standalone sentence like this, the default interpretation is the speaker’s expectation.

おいしい is an i-adjective, and its past form is おいしかった.

• おいしい = delicious
• おいしかった = was delicious / tasted good

The past tense is used because the speaker is talking about the salad as something they already ate or experienced.

In English, was delicious sounds natural here because the judgment happened after tasting it.

For i-adjectives, you replace the final い with かった.

So:

• おいしい → おいしかった
• たかい → たかかった
• おもしろい → おもしろかった

That is why this sentence has おいしかった.

They are similar, but not exactly the same in nuance.

• 思ったより = than I thought
• 予想より = than expected / than my expectation

思ったより sounds very natural and conversational. It often expresses a personal reaction like:

• It was better than I thought
• It was tastier than I expected

So for this sentence, 思ったより fits very naturally.

Yes, usually there is at least a mild sense of unexpectedness.

思ったよりおいしかった suggests something like:

• It was tastier than I expected
• It was actually pretty good
• It turned out to be better than I thought

This often implies that the speaker did not expect the salad to be so tasty.

In Japanese, you do not always need a separate word for more when using より in this kind of expression.

• 思ったよりおいしかった naturally means was more delicious than I thought
• The comparison is already understood from より

Japanese often expresses comparison more compactly than English.

No. While that is very close to the literal grammar, deliciouser is not natural English.

Better translations are:

• The spinach and daikon salad was more delicious than I thought.
• The spinach and daikon salad was tastier than I expected.
• The spinach and daikon salad was better than I thought.

So the Japanese grammar is comparative, but the best English wording may vary.

The comma is normal, but the spaces usually are not.

Normally, this would be written like this:

ほうれん草と大根のサラダは、思ったよりおいしかった。

Japanese does not usually use spaces between words in standard writing. Spaces are often added only in learning materials to make the sentence easier to read.