Learn Hardcore Japanese: kinou, otouto to kenka wo sita ga, kesa mata nakanaori dekite ansinsimasita. - Yesterday I fought with my little brother, but this morning we made up again and I felt relieved.

昨日、… is a neutral way of giving the time.
昨日は… slightly emphasizes “as for yesterday” or contrasts it with other times (e.g. Yesterday (as opposed to today), I fought with him).

In this sentence, 昨日、 without a particle is natural and standard.

What does と after 弟 mean here? Is it the same と as “and”?

Here 弟と means “with my younger brother” (the person you do the action with).

弟とけんかをした
= I fought *with my younger brother.*

This と is the “with (as a counterpart)” particle, used with many verbs involving interaction:

友だちと話す – to talk with a friend
彼氏と別れる – to break up with one’s boyfriend
弟と遊ぶ – to play with my younger brother

So in this sentence, it does not mean “and A and B”; it marks your partner in the action.

Why is it けんかをした and not just けんかした? What is けんか grammatically?

けんか is a noun meaning “a fight / quarrel.”
The verb “to fight (in that sense)” is made by combining it with する:

けんかする – to fight, quarrel

In the past tense:

けんかした
けんかをした

Both are correct and common.
Here, を is marking けんか as the direct object of する, but in everyday speech けんかした (dropping を) is very common and completely natural.

So:

弟とけんかした
弟とけんかをした

= same basic meaning.

I thought が was a subject marker. Why is したが、 using が to mean “but”?

が has two main uses:

Subject marker:
- 猫がいる。 – There is a cat.
Conjunction meaning “but / although”:
- 雨が降っているが、出かける。
  It’s raining, but I’ll go out.

In your sentence:

昨日、弟とけんかをしたが、今朝また仲直りできて安心しました。

this が is the conjunction, not the subject marker. It connects:

Clause A: 昨日、弟とけんかをした (I fought with my younger brother yesterday)
Clause B: 今朝また仲直りできて安心しました (but this morning we made up again and I felt relieved)

You could also say けど or けれど(も) instead of が in many situations, though が sounds a bit more formal / written-like.

What exactly does また mean here in 今朝また仲直り? Is it “again” or “also”?

Here また means “again”:

今朝また仲直りできて…
= We were able to make up *again this morning…*

The nuance is that:

They fought yesterday, then
They made up again this morning (perhaps after being on bad terms for a while, or after a previous cycle of fighting and making up in the past).

また can also sometimes mean “also / in addition,” but in this context with 仲直り it’s naturally understood as “once more, again.”

What is 仲直り by itself? Is it a verb or a noun?

仲直り (なかなおり) is a noun that means:

reconciliation; making up (after a quarrel); restoration of a relationship.

You usually use it with する or できる:

仲直りする – to make up
仲直りできる – to be able to make up; manage to make up

So:

今朝また仲直りした。
We made up again this morning.
今朝また仲直りできた。
We were able to make up again this morning / I managed to make up with him again this morning.

Why is it 仲直りできて and not 仲直りして? What’s the difference?

Both are grammatically possible, but the nuance is different.

仲直りして安心しました。
I made up (with him) and felt relieved.
仲直りできて安心しました。
I was *able to make up (with him) and felt relieved.*

できる adds the idea of ability / successful completion and often carries a nuance of “I’m glad it worked out / I’m relieved it happened.” In emotional contexts (relief, happiness, gratitude), 〜できてよかった / 〜できて安心した is very common.

So 仲直りできて安心しました implies something like:

“I’m relieved that we managed to reconcile (that we were able to make up).”
— not just the bare fact “we made up,” but with a sense of success/relief.

What is the role of the て-form in 仲直りできて安心しました?

The て-form here connects two actions/states:

仲直りできて – (having been able to) make up
安心しました – felt relieved

In this kind of sentence, the て-form often implies a cause or reason:

仲直りできて安心しました。
= I felt relieved *because we were able to make up.*

So the structure is:

[Reason / preceding event in て-form] + [main feeling / result]
- 合格できてうれしいです。 – I’m happy because I passed.
- 早く終わって助かった。 – I was saved / glad because it finished early.

Here, 仲直りできて explains why the speaker feels relieved.

Why is 安心しました in the past tense? Isn’t the feeling “I am relieved” now?

Japanese often uses 〜しました / 〜だった for feelings that arose because of something.

安心しました。
literally: I became relieved / I felt relieved.

It describes the change into that emotional state at that moment. The English equivalent is usually present (“I’m relieved”) or present perfect (“I’ve been relieved”), but Japanese uses past.

Compare:

安心です。 – It is safe / I’m (in a state of) being not worried.
安心しました。 – I felt relief; I became relieved (when X happened).

In this sentence, because 仲直りできて is something that happened this morning, 安心しました naturally matches it as a past reaction to that event.

Could I say 今朝また仲直りして安心しました instead? Is that wrong?

It’s not wrong; it’s grammatically fine and understandable:

今朝また仲直りして安心しました。

The nuance:

仲直りして安心しました:
I made up (with him) and felt relieved. (just states the two events)
仲直りできて安心しました:
I was able to / managed to make up (with him) and felt relieved. (adds a sense of success or gratitude that it was possible)

In many real situations, speakers prefer 〜できて安心しました / 〜できてよかったです when expressing emotional relief, which is why できて sounds especially natural here.

Are there more casual or alternative ways to say this whole sentence?

Yes, you can make it more casual or change connectors slightly. For example:

昨日、弟とけんかしたけど、今朝また仲直りできてホッとした。
Yesterday I fought with my little brother, but we were able to make up again this morning and I felt relieved.

Changes here:

けんかした instead of けんかをした – slightly more casual.
けど instead of が – more spoken / casual.
ホッとした instead of 安心しました – very colloquial “I felt relieved / what a relief.”

Polite version (close to original):

昨日、弟とけんかをしましたが、今朝また仲直りできて安心しました。

This just upgrades した → しました to make it uniformly polite.