kyou ha amari tabetaku nakatta node, okazu wo hanbun nokosimasita

That’s true: time expressions like 今日 often appear with no particle at all.

• 今日食べたくなかったので… = Today, I didn’t feel like eating much, so…
• 今日は食べたくなかったので… = As for today / today in particular, I didn’t feel like eating much, so…

Here, は makes 今日 the topic and can add a slight contrastive nuance, like today was that kind of day. In natural English, this usually still just comes out as today.

あまり commonly means not very or not much when it is used with a negative.

So:

• あまり食べたくなかった = didn’t want to eat very much
• more literally: was not very desirous of eating

This is a very common pattern:

• あまり好きじゃない = don’t like it very much
• あまり分からない = don’t really understand it very well

So in this sentence, あまり softens the statement a little: the speaker is saying they were not in the mood to eat much, not that they absolutely did not want to eat at all.

It comes from the verb 食べる and the たい form.

Step by step:

1. 食べる = to eat
2. 食べたい = want to eat
3. 食べたくない = don’t want to eat
4. 食べたくなかった = didn’t want to eat

A useful point: たい behaves like an い-adjective, so it conjugates like one.

• 〜たい
• 〜たくない
• 〜たかった
• 〜たくなかった

That is why you get 食べたくなかった, not something like 食べたいなかった.

They mean different things.

• 食べなかった = didn’t eat
• 食べたくなかった = didn’t want to eat

The sentence is talking about the speaker’s desire/appetite, not simply the fact of eating or not eating.

That matters because the second half says they left half of the side dishes. So the idea is:

• they didn’t feel like eating much,
• therefore they left some food.

If you said 食べなかったので, it would sound more like because I didn’t eat..., which is a different idea.

ので means because / since, and it connects the reason to the result:

• あまり食べたくなかったので、…残しました
• Because I didn’t feel like eating much, I left half of the side dishes.

Compared with から, ので often sounds a bit softer, more explanatory, and a little less direct.

Very roughly:

• から: can sound more straightforward, personal, or assertive
• ので: often sounds more gentle, neutral, or matter-of-fact

Both could work here, but ので feels very natural in this kind of explanation.

The subject is omitted because it is understood from context.

In English, you usually need to say I, but in Japanese, if the speaker is obviously talking about themself, 私 is often left out.

So this sentence naturally means something like:

• I didn’t feel like eating much today, so I left half of the side dishes.

Japanese very often omits subjects and other information when they are clear from context.

おかず usually means the dishes eaten along with the staple food, especially rice. It is often translated as:

• side dish
• dish
• food to go with rice

In a meal context, it refers to the non-rice part of the meal, such as meat, fish, vegetables, etc.

So おかずを半分残しました means the speaker left half of those accompanying dishes uneaten.

Here:

• おかずを marks おかず as the direct object of 残しました
• 半分 tells you how much of it was left

So the meaning is:

• left half of the side dishes
• or more naturally, left half of the food/side dishes uneaten

Japanese often allows this kind of structure:

• パンを少し食べた = ate a little of the bread
• ご飯を半分残した = left half the rice

You may also hear おかずを半分だけ残しました, where だけ adds the nuance of only.

That would mean something different.

• おかずを半分残しました = left half of the side dishes
• 半分のおかずを残しました = left the half portion of the side dishes or left half the side dishes depending on context, but it sounds less natural for this situation

In this sentence, 半分 is not really describing which side dishes. It is describing how much of the side dishes was left. So おかずを半分残しました is the natural phrasing.

You could also say:

• おかずの半分を残しました

That more explicitly means I left half of the side dishes.

The key point is that 残す is transitive and 残る is intransitive.

• 残す = to leave something
• 残る = to remain / be left

So:

• おかずを半分残しました = I left half of the side dishes
• おかずが半分残りました = Half of the side dishes remained

The sentence uses 残しました because the speaker is describing their own action: they left food uneaten.

Yes, that is completely normal.

In Japanese, the politeness level is usually controlled mainly by the final verb of the sentence. A clause before ので, から, けど, etc. is often in plain form even when the whole sentence is polite.

So this pattern is very common:

• 時間がなかったので、先に帰りました。
• あまり眠くなかったので、まだ勉強しました。

Here:

• 食べたくなかった = plain form inside the reason clause
• 残しました = polite past at the end of the sentence

That combination is natural and standard.

Because the sentence is describing a completed situation in the past.

• 食べたくなかった = the speaker did not feel like eating
• 残しました = the speaker left the food

Both happened as part of one past event, so both are in past form.

If you changed the tense, the meaning would change too. For example:

• 今日はあまり食べたくないので、おかずを半分残します。 = I don’t feel like eating much today, so I’ll leave half of the side dishes.

That sounds like something happening now or about to happen.

It is not strictly required, but it is very common and helpful.

The comma separates:

• the reason clause: 今日はあまり食べたくなかったので
• the main result clause: おかずを半分残しました

Japanese punctuation is somewhat flexible, and you may see the sentence written without a comma too. But with a longer clause like this, the comma makes it easier to read.

No, normal Japanese writing usually does not use spaces between words.

The sentence would normally be written like this:

今日はあまり食べたくなかったので、おかずを半分残しました。

Spaces are often added in teaching materials to help learners see the word boundaries. So the spaced version is just a learning aid, not standard Japanese formatting.