koohii ga sukosi nigai node, gyuunyuu wo iremasu.

Here, が marks コーヒー as the thing being described by 苦い.

• コーヒーが苦い = the coffee is bitter
• The adjective 苦い is saying something about coffee, so coffee gets が.

A learner might expect は, and コーヒーは少し苦いので… would also be possible. But が is natural when you are simply identifying what is bitter in this situation.

少し means a little or slightly.

It modifies 苦い:

• 少し苦い = a little bitter / slightly bitter

So the sentence is not saying the coffee is strongly bitter, just somewhat bitter.

苦い is an i-adjective.

Its dictionary form is 苦い and it means bitter.

Examples:

• 苦いコーヒー = bitter coffee
• コーヒーが苦い = the coffee is bitter

Because it is an i-adjective, it can directly end a clause like this without needing だ.

Because ので attaches to the plain form of the word before it.

For an i-adjective, the plain form is the form ending in -い:

• 苦い
  • ので → 苦いので

苦く is a different form used in other grammar patterns, such as:

• 苦くない = not bitter
• 苦くて = bitter, and...
• 苦くなる = become bitter

So 苦くので is not correct here.

ので means because, since, or so.

In this sentence:

• コーヒーが少し苦いので、牛乳を入れます。
• Because the coffee is a little bitter, I add milk.

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

This is very normal in Japanese.

The clause before ので usually uses the plain form:

• 苦いので

The main sentence can still be polite:

• 入れます

So this mix is natural:

• plain form inside the reason clause
• polite form at the end of the whole sentence

You could also hear very formal speech like 苦いですので, but that is less common in everyday Japanese.

を marks the direct object of the verb 入れます.

So:

• 牛乳を入れます = add milk / put in milk

The milk is the thing being added, so it takes を.

入れます literally means put in or insert, but in this context it is best understood as:

• add
• put in

So 牛乳を入れます means I add milk or I put milk in.

Because the sentence is about coffee, the natural interpretation is I add milk to the coffee.

It is omitted because the destination is obvious from context.

The full idea could be:

• コーヒーに牛乳を入れます。
• I put milk into the coffee.

But since the sentence already starts by talking about the coffee, Japanese often leaves コーヒーに out.

This omission is very common when the meaning is clear.

Yes. Japanese often omits subjects when they are understood from context.

So this sentence naturally means something like:

• Because the coffee is a little bitter, I add milk.

But 私は is not necessary unless you want to emphasize I.

Yes, you could say:

• コーヒーが少し苦いから、牛乳を入れます。

This also means Because the coffee is a little bitter, I add milk.

The difference is mainly nuance:

• ので = softer, more explanatory
• から = a bit more direct, casual, assertive

Both are correct.

Yes, in casual speech that is very common:

• コーヒーがちょっと苦いので、牛乳を入れます。

ちょっと and 少し can both mean a little.

The difference:

• 少し sounds a bit more neutral or textbook-like
• ちょっと sounds more conversational

The spaces are usually just for learners.

Normal Japanese writing would be:

• コーヒーが少し苦いので、牛乳を入れます。

Japanese normally does not use spaces between words the way English does. They are often added in teaching materials to make the structure easier to see.