tumetai mizu ni remon wo irete nomu to, asa ha kibun ga ii desu.

Because 冷たい is an i-adjective, and i-adjectives can directly modify nouns.

So:

• 冷たい水 = cold water
• 大きい犬 = a big dog
• 新しい本 = a new book

There is no particle needed between the adjective and the noun.

In this sentence, 冷たい水 is just one noun phrase: cold water.

Because with 入れる, the thing being put in takes を, and the place/container it goes into takes に.

So here:

• レモンを = the lemon is the thing being added
• 水に = the water is what it is being added to

That is why:

• 水にレモンを入れる = put lemon into water

If you said 水をレモンに入れる, it would mean put water into a lemon, which is not what this sentence means.

Here, に marks the destination or target location of the action.

With 入れる, に often means into or in.

Examples:

• コップに水を入れる = put water into a cup
• 袋に本を入れる = put a book into a bag
• 水にレモンを入れる = put lemon into water

So in this sentence, に is doing the job that into does in English.

The て-form connects actions. Here it means something like:

• put in, and then drink
• putting in and drinking

So:

• レモンを入れて飲む = put in lemon and drink

This is a very common Japanese pattern for action sequences:

• 起きて、顔を洗う = wake up and wash one’s face
• 店に行って、パンを買う = go to the store and buy bread
• レモンを入れて飲む = add lemon and drink

The て-form often implies that the first action happens before the second.

No, と here is not and. It is a conditional marker, often meaning:

• when
• if
• whenever

So:

• レモンを入れて飲むと、朝は気分がいいです。

means something like:

• When I drink it with lemon added, I feel good in the morning.
• If I drink cold water with lemon, I feel good in the morning.

This と often describes something that happens naturally, regularly, or as a result.

A very common example:

• 春になると、暖かくなる。 = When spring comes, it gets warm.

So in this sentence, 飲むと suggests a repeated or habitual result.

Because は is marking 朝 as the topic: as for the morning or in the morning.

So 朝は気分がいいです has the feeling of:

• As for the morning, I feel good
• In the morning, I feel good

Using 朝に would focus more on a specific time point, and it sounds less natural in this kind of general statement.

Compare:

• 朝は気分がいいです = speaking generally about mornings
• 朝に起きます = I wake up in the morning

In your sentence, 朝は is natural because the speaker is talking about how things are in the morning as a general situation.

Because 気分がいい is a very common expression meaning:

• feel good
• be in a good mood
• feel well

Here, 気分 is the thing being described as いい, so が marks it naturally.

• 気分がいい = the feeling/mood is good

If you said 気分はいい, it would sound more contrastive, like:

• As for my mood, it’s good
  maybe implying that something else is not good

So:

• 気分がいい = neutral, standard expression
• 気分はいい = contrast/emphasis

That is why が is the normal choice here.

It can mean a few related things depending on context.

気分がいい can mean:

• I feel good
• I’m in a good mood
• I feel refreshed
• sometimes I feel physically well

In this sentence, because it is about drinking cold water with lemon and then how one feels in the morning, it likely means something like:

• I feel good in the morning
• I feel refreshed in the morning

So the English translation may vary a little depending on context, but the Japanese expression itself is very natural.

Because Japanese often leaves out the subject when it is understood from context.

In English, we usually need to say:

• I feel good
• You feel good
• People feel good

But in Japanese, if the speaker is talking about their own habit or a general experience, the subject can be omitted.

So this sentence could imply:

• I
• you
• one
• people in general

depending on the situation.

This is very normal in Japanese. Omitting obvious information makes the sentence sound more natural, not incomplete.

いい and よい are basically the same word.

• よい is the dictionary/base form
• いい is the very common everyday form

So:

• 気分がいい = normal, natural speech
• 気分がよい = more formal or stiff

Learners should also know that this adjective is a little irregular in conjugation. Even though we often say いい, many changed forms come from よい:

• よくない = not good
• よかった = was good
• よく = well

So in the sentence, いいです is the natural present polite form.

Yes, in meaning it is very close.

Literally, 冷たい水にレモンを入れて飲む means:

• put lemon into cold water and drink it

In natural English, we would often simplify that to:

• drink cold lemon water
• drink water with lemon in it

Japanese sometimes expresses the action more step-by-step than English does. So even though the literal wording is longer, the meaning is straightforward.

The sentence breaks down like this:

• 冷たい水に = into cold water
• レモンを = lemon
• 入れて = put in, and
• 飲むと = when/if (I) drink
• 朝は = in the morning / as for mornings
• 気分がいいです = (I) feel good

So the overall pattern is:

[Action] と、 [result]

More specifically:

When/if I put lemon in cold water and drink it, I feel good in the morning.

That is a very common Japanese way to express a habitual result.