tomodati to yakusokusita kara, ame ga hutte mo ikanai wake ni ha ikanai.

行かないわけにはいかない is a very common pattern meaning:

• I have to go
• I can’t just not go
• I have no choice but to go

It looks confusing because it contains two negatives:

• 行かない = not go
• ～わけにはいかない = cannot do such a thing / cannot allow oneself to do that

So literally, it is something like:

• It won’t do to not go
• I can’t choose not to go

This pattern often expresses a social, moral, or situational pressure rather than a simple neutral obligation.

These mean almost opposite things.

• 行かないわけにはいかない = I have to go
• 行くわけにはいかない = I can’t go

Why?

• In 行かないわけにはいかない, the negative verb 行かない is inside the pattern.
  The whole idea is I cannot choose the option of not going.
• In 行くわけにはいかない, the verb is positive, so it means I cannot do the act of going.

So:

• 行かないわけにはいかない → not going is not acceptable
• 行くわけにはいかない → going is not acceptable

This is one of the most important things to notice in this sentence.

Yes, 約束した comes from 約束する, which means:

• to promise
• to make an arrangement
• to agree on something

In this sentence, it most naturally means something like:

• I made plans with a friend
• I promised a friend
• I arranged to meet a friend

So the reason the speaker feels they must go is that they already made a commitment.

Here, と marks the person you made the promise or arrangement with.

So:

• 友達と約束した = I made a promise/arrangement with a friend

This と is the same basic with idea seen in many other expressions:

• 友達と話す = talk with a friend
• 友達と行く = go with a friend

With 約束する, と is very natural when the promise is an arrangement involving both people.

から means because here.

So the sentence is divided like this:

• 友達と約束したから = because I promised / made plans with a friend
• 雨が降っても行かないわけにはいかない = even if it rains, I have to go

So から introduces the reason for the speaker’s feeling of obligation.

雨が降っても means:

• even if it rains
• even though it rains in some contexts

Breakdown:

• 雨 = rain
• が = subject marker
• 降って = te-form of 降る , to fall / to rain
• も = even if / even

The pattern te-form + も often means even if:

• 行っても = even if I go
• 食べても = even if I eat
• 雨が降っても = even if it rains

So the speaker is saying the obligation remains true despite bad weather.

In 降っても, the も adds the meaning of even if.

This is a standard grammar pattern:

• Verb て-form + も
• い-adjective くて + も
• na-noun / noun + でも

Examples:

• 忙しくても行く = I’ll go even if I’m busy
• 静かでも大丈夫 = It’s fine even if it’s quiet
• 雨が降っても行く = I’ll go even if it rains

So in your sentence, 降っても presents a condition that would normally stop someone, but here it does not.

Japanese often leaves out subjects when they are understood from context.

In this sentence, the subject is most naturally:

• I

So the full idea is something like:

• Because I promised a friend, even if it rains, I have to go.

Japanese does this very often. If the speaker is talking about their own situation, 私は is usually omitted unless there is a reason to emphasize it.

They are similar, but not exactly the same.

• 行かなければならない = I must go
• 行かなくてはいけない = I have to go
• 行かないわけにはいかない = I can’t not go / I have no choice but to go

The first two are more general ways to say obligation.

～ないわけにはいかない often has a stronger nuance of:

• social pressure
• moral obligation
• emotional pressure
• circumstances making refusal difficult

In this sentence, that nuance fits well:
the speaker already made a promise to a friend, so not going would feel unacceptable.

雨が降る is the normal way to say it rains.

Here, 雨 is the thing that does the action 降る, so が marks it as the subject.

• 雨が降る = rain falls / it rains

You could sometimes see 雨は in other contexts if the speaker wants to contrast or topic-mark rain, but 雨が降っても is the straightforward, natural form here.

Yes, most likely.

Even though 約束した is past tense, that part refers to the completed action of making the promise:

• I promised / made plans

The main action, 行かないわけにはいかない, refers to what the speaker must do now or in the future:

• I have to go

So the timeline is:

1. I made plans with a friend.
2. Therefore, even if it rains, I have to go.

Yes, that is a very good natural translation.

Other natural translations include:

• Because I made plans with a friend, I have to go even if it rains.
• I promised a friend, so I can’t just not go, even if it rains.
• Since I made a promise to a friend, I have no choice but to go, even if it rains.

All of these capture the same core idea. The exact English wording depends on how directly you want to reflect the Japanese grammar.

The overall nuance is:

• the speaker does not necessarily want to go under bad conditions
• but they feel they should go because they made a promise

So it sounds like:

• I’d rather not go if it rains, but I really ought to because I already promised.

This feeling of reluctant obligation is a key part of 行かないわけにはいかない.