watasi mo, sono udon wo okawarisitakatta desu ga, onaka ga ippai desita.

も means also or too.

So 私も means I too or me too. It suggests that someone else also wanted another serving, and the speaker is adding themselves to that group.

Compare:

• 私は = as for me
• 私も = I too / I also

In this sentence, も adds the nuance I wanted more udon too, but...

その means that and points to something close to the listener, or something already mentioned in the conversation.

So そのうどん means that udon.

A learner often compares:

• この = this
• その = that
• あの = that over there

Here, そのうどん probably refers to a specific udon that the listener knows about, or that was just mentioned.

おかわり means another helping, seconds, or a refill.

It is a noun, and it often appears in expressions like:

• おかわりする = to have another helping / to get seconds
• おかわりください = seconds, please / another serving, please

The お here is an honorific/polite prefix. In everyday Japanese, おかわり is a very common word when talking about food and drinks.

So うどんをおかわりしたかった means the speaker wanted another serving of the udon.

The particle を marks the direct object of the verb-like expression おかわりする.

In うどんをおかわりする, the thing being repeated or taken again is udon, so うどん gets を.

It may feel slightly unusual to English speakers because おかわり is a noun, but おかわりする functions like a verb phrase. So the structure is:

• うどん = the thing
• を = object marker
• おかわりする = to have another helping

So the whole part means to have another helping of udon.

It breaks down like this:

• おかわりする = to have another helping
• おかわりしたい = want to have another helping
• おかわりしたかった = wanted to have another helping

The key part is ～たい, which expresses desire: want to do.

Then its past form ～たかった means wanted to do.

So:

• する → do
• したい → want to do
• したかった → wanted to do

In this sentence, the speaker is talking about a past feeling, so したかった is the correct form.

Yes, したかったです is completely normal.

The ～たい form behaves like an i-adjective, and polite speech often adds です after it:

• 食べたいです = I want to eat
• したかったです = I wanted to do

So this is not a mistake. It is a common polite way to say it.

The sentence combines:

• おかわりしたかったです = I wanted another helping
• が = but

So the first clause is polite, and then it connects to the next clause.

Here, が does not mean the subject marker. It means but.

Japanese が has more than one use:

• as a particle marking the subject
• as a conjunction meaning but

In this sentence, it connects two clauses:

• おかわりしたかったです = I wanted another helping
• お腹がいっぱいでした = I was full

So the meaning is:

I wanted another helping, but I was full.

This use of が often sounds a little softer than a strong English but.

This が is the subject marker.

The phrase お腹がいっぱい literally means the stomach is full. In natural English, we usually translate that as I am full.

So the structure is:

• お腹 = stomach / belly
• が = subject marker
• いっぱい = full
• でした = was

A very literal translation would be:

My stomach was full.

But the natural English meaning is just:

I was full.

Here, いっぱい means full.

In other contexts, いっぱい can also mean things like:

• a lot
• to the limit
• all full

But in the expression お腹がいっぱい, it is a very common set phrase meaning to be full after eating.

So when learners see いっぱい, they should pay attention to context. In this sentence, it does not mean a lot in a vague sense; it specifically means full.

Because the speaker is describing a past situation.

• おかわりしたかったです = wanted another helping
• いっぱいでした = was full

This tells us that both the desire and the reason belonged to that earlier moment.

If you changed it to present tense, it would mean the situation is true now:

• おかわりしたいです = I want another helping
• お腹がいっぱいです = I am full

But the given sentence is about how the speaker felt at that time.

Yes, very often.

Japanese frequently leaves out the subject when it is clear from context. So a very natural version could be:

そのうどんをおかわりしたかったですが、お腹がいっぱいでした。

Or in even more casual speech:

そのうどん、おかわりしたかったけど、お腹いっぱいだった。

Japanese does not need to say I as often as English does. In the original sentence, 私も is included for emphasis, especially to mean I too.