Learn Hardcore Chinese: Jīntiān wǒ de xīnqíng bù tài hǎo.

心情不太好 literally means "(my) mood not too good" and already works as a complete predicate.
The structure is: (Time) + (Possessor) + Noun + Adjective phrase
→ 今天 + 我的 + 心情 + 不太好

You can say 今天我的心情不是太好, but:

It sounds a bit heavier or more formal.
不是太好 often feels like "is really not that good", with a bit more emphasis.

For everyday speech, 今天我的心情不太好 is the most natural.

What does the particle 的 in 我的心情 do? Why not just say 我心情?

的 is a possessive/attributive particle. It connects a possessor or modifier to a noun.

我 = I / me
的 = possessive marker
心情 = mood

So 我的心情 = my mood.

In general:

A + 的 + Noun → A’s Noun
- 我的书 = my book
- 他的朋友 = his friend

Here, 我的心情 = my mood.

Can I drop 的 and say 今天我心情不太好? Is it still correct?

Yes, you can say 今天我心情不太好, and it’s common in casual speech.

Nuance:

今天我的心情不太好
- Slightly more careful/explicit; 我的 clearly marks possession.
今天我心情不太好
- A bit more colloquial and quick; 我 directly modifies 心情 without 的.

With personal pronouns like 我 / 你 / 他, 的 is often optional before a noun that clearly belongs to that person:

我(的)妈妈, 他(的)手机, 我(的)房间

So both versions are natural.

Why does 今天 come at the beginning? Could I say 我今天的心情不太好 instead?

Both are correct:

今天我的心情不太好
我今天的心情不太好

Differences are subtle:

今天我的心情不太好
- Puts more topic focus on 今天 (today): As for today, my mood isn’t very good.
我今天的心情不太好
- Puts more focus on 我 (I): As for me, today my mood isn’t very good.

Chinese often puts time words like 今天 near the beginning of the sentence:

今天我很忙。
明天我们去北京。

Both orders are natural; it’s mostly a matter of what you want to emphasize first.

What exactly does 心情 mean? Is it the same as feeling or emotion?

心情 (xīnqíng) means mood, your general emotional state at a certain time.

It’s more about the overall mood, not a specific emotion.
Roughly: state of mind / mood.

Examples:

心情很好 – (in a) very good mood
心情不好 – (in a) bad mood
影响心情 – affect one’s mood

It’s closer to English mood than to a specific feeling like happy or sad.

What does 不太好 mean exactly? Is it "not very good", "too not good", or something else?

不太好 (bù tài hǎo) usually means "not very good" or "not so good"—a mild negative.

Breakdown:

太好 by itself: too good / very good (often "too" as in "excessively")
不太好:
- Literally: "not too good"
- Natural meaning: "not very good / not so good"

It sounds softer and less harsh than:

不好 – "bad / not good" (more direct, stronger)
很不好 – "very bad" (even stronger)

So 今天我的心情不太好 feels like:

"My mood isn’t so good today", not "My mood is terrible."

Why is 不 placed before 太 (不太好) instead of before 好 (like 太不好)?

不太 + adjective is a set pattern meaning "not very + adjective."

不太好 – not very good
不太忙 – not very busy
不太舒服 – not very comfortable

If you say 太不好, it has a different meaning:

太 + 不好 ≈ "too not good / really bad" (stronger negative)
太不好了 – this is awful / terrible.

So:

不太好 = mild: not so good
太不好 = strong: way too bad

In the sentence, the speaker is downplaying how bad the mood is, so 不太好 is appropriate.

How should I pronounce 不太 (不 + 太)? I learned that 不 is fourth tone, but this sounds different.

In isolation, 不 is fourth tone: bù.

But there is a tone sandhi rule:

When 不 (bù) is followed by another fourth tone, it changes to second tone bú.

太 (tài) is fourth tone, so:

不太 is pronounced bú tài, not bù tài.

Other examples:

不对 → bú duì
不敢 → bú gǎn

In writing/pinyin you often still see bù, but you should pronounce it as bú before a fourth tone.

Could I replace 心情不太好 with 我很不高兴? Do they mean the same thing?

They’re related but not identical.

心情不太好
- Literally: (my) mood is not very good
- Vague, soft, can cover sadness, irritation, stress, etc.
- Sounds more neutral and polite, less dramatic.
我很不高兴
- Literally: I am very unhappy / very displeased
- Stronger and more specific: you are unhappy / upset.
- Could imply you’re annoyed or angry at something.

So:

If you just want to say you’re in a bad mood in general:
→ 今天我的心情不太好。
If you want to clearly say you’re unhappy / upset (maybe at someone or something):
→ 我很不高兴。

Is 心情 countable in Chinese? Can I say something like 一個心情?

In everyday modern Chinese, 心情 is usually treated as uncountable—you don’t normally say 一个心情 in the sense of "one mood."

You typically see:

心情不好 / 心情很好 – mood is bad/good
有心情做… – to be in the mood to do…
没有心情聊天 – not in the mood to chat

You can see 各种心情 (all kinds of moods) or 复杂的心情 (complicated feelings), but this still treats 心情 more as a mass noun than as discrete countable items with 一个/两个.

I learned that descriptions use 很 (like 我很忙, 他很好). Why is there no 很 here? Could I say 今天我的心情不太好很?

You cannot say …不太好很; 很 goes before the adjective, not after it.

Pattern:

(Subject) + 很 + Adjective
- 我很忙。 – I am busy.
- 他很好。 – He is very good.

When you already have another adverb like 太, you don’t normally add 很:

不太好 is already an adverb + adjective phrase.
Adding 很 would be ungrammatical: 不太很好, 不太好很 – both wrong.

You could say:

今天我的心情很好。 – My mood is very good today.
今天我的心情不太好。 – My mood is not very good today.

But you don’t combine 很 with 不太 in this context.

Are there other common ways to say "My mood is not so good today" in Chinese, and how do they differ from 心情不太好?

Yes, here are some common alternatives:

今天我心情不好。
- Stronger than 不太好.
- Closer to "My mood is bad today."
今天我有点儿不开心。
- 有点儿 = a little bit (often for negative things)
- 不开心 = unhappy
- Feels softer and more casual: "I’m feeling a bit unhappy today."
今天我有点儿烦。
- 烦 = annoyed / irritated
- More specific: you feel annoyed, bothered.
今天我状态不太好。
- 状态 = state/condition
- Can refer to emotional or physical state; good for talking about performance or energy.

Among these, 今天我的心情不太好 is a polite, neutral, and slightly soft way to say you’re not in a great mood.