wǒ juéde zhè jiàn shìqíng bú tài jiǎndān.

觉得 (juéde) often means to think, to feel, or to find something to be a certain way.

In this sentence, 我觉得…… is a very common way to say:

• I think...
• I feel that...
• It seems to me that...

So 我觉得这件事情不太简单 means something like:

• I think this matter isn’t so simple.
• I feel this situation is not that simple.

Compared with 我想, 我觉得 often sounds more like a personal judgment or impression.

The sentence follows this pattern:

我 + 觉得 + [clause]

So here:

• 我 = I
• 觉得 = think / feel
• 这件事情不太简单 = this matter is not very simple

You can think of it as:

I think + this matter is not that simple.

A simple breakdown:

• 我 — subject
• 觉得 — opinion verb
• 这件事情 — topic/thing being discussed
• 不太简单 — description/judgment

件 (jiàn) is a measure word.

In Mandarin, when you use a demonstrative like 这 (this) or 那 (that) with many nouns, you usually need a measure word in between.

So:

• 这 + 件 + 事情
• this + measure word + matter

That is why you cannot normally just say 这事情 in standard careful Mandarin.

Common pattern:

• 这本书 = this book
• 那个人 = that person
• 这件事情 = this matter / this affair

For 事情, 件 is a very common measure word because it refers to an event, matter, or affair.

Because 件 is only a measure word; it does not replace the noun.

So:

• 这 = this
• 件 = measure word
• 事情 = matter / affair / thing

You need the actual noun after the measure word.

Compare in English:

• You would not say only this item if you really mean this matter unless the context is extremely clear.
• In Mandarin, the structure normally needs both the measure word and the noun.

That said, in casual speech people may shorten:

• 这件事 = this matter
• 这事 = this thing / this matter

These are very common too.

Both can refer to a matter, affair, or thing.

• 事情 is a bit fuller and slightly more formal/neutral.
• 事 is shorter and very common in everyday speech.

So these are all natural:

• 这件事情不太简单。
• 这件事不太简单。
• 这事不太简单。 (more colloquial)

In many situations, 这件事 is the most common everyday version.

Literally, yes, it looks like not too simple:

• 不 = not
• 太 = too / very
• 简单 = simple

But in actual usage, 不太 + adjective usually means:

• not very...
• not so...
• not particularly...

So 不太简单 usually means:

• not very simple
• not so simple
• more complicated than it seems

It does not usually mean too simple.

This is a very important pattern:

• 不太好 = not very good
• 不太清楚 = not very clear
• 不太容易 = not very easy

So 这件事情不太简单 often suggests there is more complexity than one might first think.

They are similar, but the nuance is different.

不太简单

This sounds more moderate and natural in many situations:

• not very simple
• not so simple
• it’s a bit complicated

很不简单

This is stronger:

• really not simple
• far from simple
• sometimes even quite remarkable depending on context

For example:

• 这件事情不太简单。
  = This matter is not so simple.

• 这件事情很不简单。
  = This matter is really not simple / quite complicated.

So the original sentence sounds a bit softer and more conversational.

This is because of a common tone change rule.

不 is normally fourth tone: bù.

But when 不 comes before another fourth-tone syllable, it changes to second tone:

• 不太 → bú tài
• 不是 → bú shì
• 不要 → bú yào

Since 太 (tài) is fourth tone, 不 changes from bù to bú.

So in this sentence:

• written: 不太
• pronounced: bú tài

In 觉得, the second syllable 得 is often pronounced in a light, unstressed way.

So although you may see it written as juéde, the de is often weak in real speech.

This is common in many two-syllable words in Mandarin, especially very frequent ones.

So:

• 觉得 is not usually pronounced with equal stress on both syllables
• it often sounds like juéde with a lighter second syllable

This is normal and natural.

Yes. 简单 can mean both simple and sometimes easy, depending on context.

But with 事情 here, simple is the more natural meaning.

So:

• 这件事情很简单。
  = This matter is simple.

It can also imply easy to deal with, but the core idea here is that the matter is not straightforward.

If you specifically want to emphasize easy/difficult, Mandarin often uses:

• 容易 = easy
• 难 = difficult

For example:

• 这件事情不容易。 = This matter is not easy.
• 这件事情不太简单。 = This matter is not so simple.

These are close, but not exactly the same.

Because Mandarin does not always need a verb like to be in the same places English does.

In English, we say:

• This matter is not simple.

In Mandarin, adjectives can often act like predicates by themselves.

So:

• 这件事情不太简单。
• literally: This matter not very simple.

That is a complete sentence in Mandarin.

You would not normally say:

• 这件事情是不太简单。

The verb 是 is usually not used before adjectives like 简单 in this kind of sentence.

Yes, but the tone changes a bit.

我觉得……

• very common in speech
• personal, natural, conversational

我认为……

• more formal
• more deliberate, like I believe or I hold the view that...

So:

• 我觉得这件事情不太简单。
  = very natural in everyday conversation

• 我认为这件事情不太简单。
  = more formal or analytical

Both are correct.

It can do both, depending on context.

On the surface, it simply means:

• I think this matter is not so simple.

But in real usage, it often suggests something like:

• There’s more going on here than it seems.
• This may be complicated.
• Something about this situation is not straightforward.

So yes, it can carry a slight feeling of suspicion, doubt, or hidden complexity, even though the wording itself is not dramatic.

Yes, very easily.

Common shorter versions include:

• 我觉得这件事不太简单。
• 我觉得这事不太简单。

These sound very natural in conversation.

The full version 这件事情 is still correct and natural, but a bit longer. In daily speech, many people would prefer 这件事.