imouto ga gakkarisite ita node, watasi ha daizyoubu da yo to itte nagusameta.

In 妹ががっかりしていたので, 妹が marks younger sister as the subject of the clause was disappointed.

Using が here is very natural because:

• it introduces who was in that emotional state
• it keeps the focus on the sister inside the reason clause

If you changed it to 妹は, it would sound more like you were setting up as for my sister..., which gives a slightly different contrastive/topic feeling.

So in this sentence, が is the straightforward subject marker for the subordinate clause.

がっかりする means to be disappointed.

Here, がっかりしていた is the te-form + いる pattern in the past:

• がっかりしている = is disappointed / is in a disappointed state
• がっかりしていた = was disappointed / was in a disappointed state

This often describes an ongoing state or condition in the past, not just a one-time moment of becoming disappointed.

So:

• がっかりした can sound more like became disappointed
• がっかりしていた emphasizes that she was in that disappointed state when the speaker responded

ので connects two parts of the sentence and means because or since.

Structure:

• 妹ががっかりしていたので = because my younger sister was disappointed
• 私は大丈夫だよと言って慰めた = I said It’s okay / I’m okay and comforted her

Compared with から, ので often sounds:

• a bit softer
• a bit more explanatory
• slightly less direct

So ので is a very natural choice for giving the reason here.

Yes, 私は could be omitted if the subject is clear from context.

Japanese often leaves out subjects when they are obvious. So a version without 私は would still work:

• 妹ががっかりしていたので、大丈夫だよと言って慰めた。

Including 私は does a few possible things:

• makes the speaker explicit
• helps contrast the sister and the speaker
• makes the sentence easier to follow, especially for learners or in written explanation

So it is not strictly necessary, but it is perfectly natural.

大丈夫 is a very common expression meaning something like okay, fine, all right, or no problem, depending on context.

In 大丈夫だよ:

• だ is the plain copula
• よ adds an assertive or reassuring tone, like I’m telling you or really

So 大丈夫だよ sounds like a casual reassurance.

Because the speaker is talking to their younger sister, the plain style is very natural. A more polite version would be something like 大丈夫ですよ, but that would sound less natural in a close family situation unless the relationship called for politeness.

The particle と marks the content of a quotation before verbs like 言う.

So:

• 大丈夫だよ = the words that were said
• と = marks those words as a quote
• 言って = saying

In other words, 大丈夫だよと言って means:

• saying, It’s okay
• or more naturally, by saying, It’s okay

This is one of the most common uses of と in Japanese.

言って is the te-form of 言う. The te-form connects actions.

So 大丈夫だよと言って慰めた means something like:

• I said, It’s okay, and comforted her
• or more naturally, I comforted her by saying, It’s okay

The te-form here shows that the actions are linked. It can imply:

• sequence: first saying it, then comforting
• means/manner: comforting her through those words

言った慰めた would not be grammatical as a normal connection of two verbs.

The person being comforted is understood to be the younger sister from context.

慰める is usually used with an object, so you could say:

• 妹を慰めた

But Japanese often omits things that are obvious. Since the sentence already talks about the sister being disappointed, listeners naturally understand that she is the one being comforted.

So the omitted object is very normal here.

Grammatically, Japanese often leaves that kind of detail implicit, and context decides it.

In this sentence, 大丈夫だよ is being used as reassurance. Depending on the situation, it could feel like:

• I’m okay
• It’s okay
• Don’t worry
• Everything’s fine

That flexibility is very common with 大丈夫. The exact English wording changes based on context, even though the Japanese stays the same.

It breaks down like this:

• 妹ががっかりしていたので
  because my younger sister was disappointed

• 私は
  I

• 大丈夫だよと
  It’s okay / I’m okay, quoting the words spoken

• 言って
  saying

• 慰めた
  comforted

So the full structure is:

[Reason clause] + [main action]

More specifically:

Because my younger sister was disappointed, I comforted her by saying, It’s okay.

That is a very common Japanese pattern:

• reason/explanation first
• main action after that