kono zyamu ha omotta hodo amaku nai node, bataa to issyo ni tabete mo oisii desu.

Because it refers to what the speaker thought beforehand, before actually trying the jam.

In this pattern, 思ったほど〜ない is very common and means not as ~ as I thought.

So the past tense does not mean the sweetness was in the past. It means the expectation was earlier.

• 思ったほど甘くない = not as sweet as I thought

You may sometimes see 思うほど, but in this kind of comment about a result or impression, 思ったほど is more natural.

Here, ほど means to the extent of or as much as.

In the pattern:

• X ほど Y ない

it means:

• not as Y as X

So:

• 思ったほど甘くない

means:

• it isn't as sweet as I thought

This is a very useful grammar pattern. For example:

• 見たほど大きくない = It's not as big as it looked
• 聞いたほど難しくない = It's not as difficult as I heard

Because 甘い is an i-adjective, and the plain negative form of an i-adjective is made like this:

• remove い
• add くない

So:

• 甘い → 甘くない

This means not sweet.

Other examples:

• 高い → 高くない = not expensive / not high
• おもしろい → おもしろくない = not interesting

Because in Japanese, the main clause usually determines the politeness level of the sentence, while subordinate parts often stay in the plain form.

So this is completely normal:

• 思ったほど甘くないので、...おいしいです

The earlier parts are plain because they are inside a modifying or connecting structure, and the final です makes the overall sentence polite.

This is one of the most common sentence patterns in Japanese:

• plain form in the middle
• polite form at the end

Both ので and から can mean because, but they feel a little different.

• ので sounds a bit softer, more explanatory, and often slightly more formal/polite
• から can sound a bit more direct or personal

Here, ので fits well because the speaker is calmly explaining the reason:

• 思ったほど甘くないので = because it's not as sweet as I thought

So the sentence has a gentle, natural explanatory tone.

Because は marks the topic of the sentence.

The sentence is about this jam:

• このジャムは = as for this jam

That sets up this jam as the thing being described in the rest of the sentence.

It also helps explain why later Japanese can omit the object of 食べて. Once this jam has already been made the topic, you do not need to repeat it.

This expression has three parts:

• バター = butter
• と = with
• 一緒に = together

So:

• バターと一緒に = together with butter

The に makes 一緒 work adverbially, so it can describe how the jam is eaten.

You can think of 一緒に as a set expression meaning together.

Because the butter is not the main thing being eaten by itself. It is the thing eaten with the jam.

• バターと一緒に食べる = eat it together with butter

If you used を with バター, that would suggest butter is the direct object being eaten.

In this sentence, the understood thing being eaten is this jam, and butter is just the accompaniment.

Grammatically, te-form + も can mean even if or even when.

Here, it does not sound like a hypothetical warning or condition. Instead, it means something more like:

• even when eaten with butter
• it also tastes good with butter

So:

• バターと一緒に食べてもおいしいです

means that the jam still tastes good, or is also tasty, when eaten with butter.

A very natural English translation is:

• It tastes good with butter too.

Because Japanese often leaves out words that are already clear from context.

The omitted object is このジャム.

So the sentence really means something like:

• このジャムは、バターと一緒に食べてもおいしいです
• As for this jam, it is delicious even when eaten with butter

Japanese does this all the time. Once the topic is established, repeating it often sounds unnecessary.

The subject of 思った is omitted.

In context, it is usually understood as the speaker:

• I thought
• sometimes we thought, depending on the situation

Japanese often omits subjects when they are obvious from context. So 思ったほど naturally means:

• not as sweet as I thought

without needing to say 私が.

It describes the result of eating this jam with butter.

So the meaning is not that butter itself is tasty. It means:

• this jam tastes good when eaten with butter
• or the combination of this jam and butter is tasty

In Japanese, this kind of sentence often leaves the exact subject slightly unstated, but the meaning is clear from context. Here, おいしいです applies to the jam / the jam-with-butter combination.