suupu ga sukosi samete kara, gyuunyuu wo mazeru to nomiyasuku naru.

Here が marks スープ as the grammatical subject of 冷めて: the soup is the thing that cools.

• スープが冷める = the soup cools
• スープは冷める would make soup the topic, often with a contrastive or explanatory feel

In this sentence, が is very natural because the speaker is simply identifying what undergoes the change.

少し means a little or slightly.

Here it modifies 冷めて, so the idea is:

• 少し冷めてから = after it cools a little / after it has cooled slightly

It does not mean a little milk. If the sentence meant a little milk, 少し would be attached to 牛乳 instead.

Because 冷める and 冷やす are different verbs:

• 冷める = to cool down, to lose heat naturally or as a state change
  
  • スープが冷める = the soup cools down
• 冷やす = to cool something, to chill something actively
  
  • スープを冷やす = to cool the soup

In this sentence, the soup itself is becoming cooler, so 冷める is the correct verb.

Verb-te form + から means after doing ...

So:

• 冷めてから = after it cools / after it has cooled

It shows sequence: first the soup cools a bit, then the next action happens.

No. This is a different use.

• 〜てから = after doing ...
• 〜から after a plain form can mean because

For example:

• 食べてから行く = I’ll go after eating
• 寒いから行かない = I won’t go because it’s cold

So in your sentence, 冷めてから definitely means after it cools, not because it cools.

を marks the direct object of 混ぜる.

So:

• 牛乳を混ぜる = mix in milk / add and mix milk

In natural English, we often say mix milk into the soup. In Japanese, the destination or target can be omitted if it is obvious from context. Here, it is understood that the milk is being mixed into the soup.

A fuller version could be:

• スープに牛乳を混ぜる

Here と is not the and/with particle. It is the conditional と, meaning when or if.

So:

• 牛乳を混ぜると = when you mix in milk / if you mix in milk

This と is often used for a natural, expected, or automatic result.

That fits well here:

• mix in milk → it becomes easier to drink

Because conditionals like 〜と use the plain form of the verb.

So:

• 混ぜると = if/when one mixes

This does not mean only a present-tense action. In Japanese, the plain non-past form is commonly used in conditionals, general statements, and instructions.

It breaks down like this:

• 飲む = to drink
• verb stem 飲み
• やすい = easy to do

So:

• 飲みやすい = easy to drink

Then:

• 飲みやすく = the adverbial form of 飲みやすい
• なる = to become

So:

• 飲みやすくなる = becomes easier to drink

This is a very common pattern:

• 読みやすい = easy to read
• 使いやすい = easy to use
• 分かりやすい = easy to understand

Because 〜やすい attaches to the verb stem, not the dictionary form.

For 飲む:

• dictionary form: 飲む
• stem: 飲み
• therefore: 飲みやすい

More examples:

• 食べる → 食べやすい
• 書く → 書きやすい
• 話す → 話しやすい

なる describes a change of state:

• 飲みやすくなる = it becomes easier to drink

This focuses on the result.

If you used する, the nuance would be more like make it easier to drink:

• 飲みやすくする = to make it easier to drink

Both are possible in Japanese, but なる is very natural here because the sentence is describing the effect of adding milk.

Yes. Japanese often omits information that is obvious from context.

Here, what is omitted is basically into the soup:

• 牛乳を混ぜる = mix in milk
• understood: into the soup

A more explicit version would be:

• スープが少し冷めてから、スープに牛乳を混ぜると、飲みやすくなる。

But repeating スープ would sound less natural than the shorter version.

It can sound like either, depending on context.

Because the sentence uses plain forms like:

• 冷めてから
• 混ぜると
• 飲みやすくなる

it has a general, recipe-like feel:

• After the soup cools a little, adding milk makes it easier to drink.

So it can work as:

• a piece of advice
• a cooking instruction
• a general statement about what happens

That general-result feeling is one reason と works well here.