wǒ gēnběn bù dǒng.

Here 根本 is an adverb meaning at all, simply, or fundamentally not in the sense of strong denial.

So 我根本不懂 means something like:

• I don’t understand it at all.
• I simply don’t understand.
• I have absolutely no understanding.

It adds emphasis. Without 根本, 我不懂 just means I don’t understand. With 根本, the speaker sounds stronger and more definite.

Mandarin word order here follows a common pattern:

• 我 = subject
• 根本 = adverb
• 不 = negation
• 懂 = verb

So the structure is:

Subject + adverb + negation + verb

This is normal in Chinese. Adverbs such as 根本, 也, 都, 常常, and 已经 usually come before the verb, and 不 also comes before the verb.

So:

• 我不懂 = I don’t understand
• 我根本不懂 = I really / absolutely don’t understand

Not exactly.

懂 means to understand, to comprehend, or to know how something works.

知道 means to know in the sense of being aware of a fact.

Compare:

• 我知道他是谁。 = I know who he is.
• 我懂这个意思。 = I understand this meaning.

So in 我根本不懂, 懂 is about understanding, not just knowing a fact.

It can also mean understanding a subject or skill:

• 你懂中文吗？ = Do you understand Chinese?
• 我不懂电脑。 = I don’t know much about computers / I don’t understand computers.

Chinese often leaves out the object when it is already clear from context.

So 我根本不懂 can mean:

• I don’t understand at all.
• I have no idea.
• I simply don’t get it.

The thing being understood might already be obvious from the conversation.

If you want to say it explicitly, you can add an object:

• 我根本不懂这个问题。 = I don’t understand this question at all.
• 我根本不懂你的意思。 = I don’t understand what you mean at all.

Very often, yes.

In everyday Mandarin, 根本 is commonly used before 不 or 没 to strongly emphasize that something is not the case:

• 我根本不懂。
• 他根本不想去。
• 这根本没用。

It can appear in other uses too, but learners will most often meet it in emphatic negative sentences.

A useful pattern is:

根本 + 不 / 没 + verb/adjective

It sounds fairly strong and emphatic.

It is stronger than:

• 我不懂。 = I don’t understand.

And similar in force to:

• 我一点也不懂。 = I don’t understand at all.
• 我完全不懂。 = I completely don’t understand.

根本 can sometimes sound a little blunt depending on tone and context. For example, if someone explains something and you say 我根本不懂, it may sound frustrated, helpless, or very direct.

They are similar, but the nuance is a little different.

• 根本不懂 = absolutely/simply don’t understand; often strong and emphatic
• 完全不懂 = completely don’t understand; focuses on total lack of understanding
• 一点也不懂 = don’t understand even a little bit; emphasizes zero amount

All three can often translate as don’t understand at all, but the feeling differs slightly.

For example:

• 我根本不懂你在说什么。 = I simply don’t understand what you’re saying.
• 我完全不懂这个理论。 = I completely don’t understand this theory.
• 我一点也不懂法语。 = I don’t understand French at all, not even a little.

Yes. Normally 不 is fourth tone: bù.

But when 不 comes before another fourth-tone syllable, it usually changes to second tone in pronunciation.

Since 懂 is third tone, not fourth tone, 不 stays fourth tone here:

• 我根本不懂 → bù dǒng

So you pronounce it as written: bù.

For comparison:

• 不是 is pronounced bú shì because 是 is fourth tone.

Yes, there is a common third-tone sandhi issue between 我 and 根.

Underlying tones:

• 我 = third tone
• 根 in 根本 = first tone
• 本 = third tone
• 不 = fourth tone
• 懂 = third tone

Because 我 is followed by a first tone, 我 usually stays a full or at least recognizable third tone.

Inside 根本, there is no third-tone sandhi because 根 is first tone and 本 is third tone.

At the end, 懂 is third tone and sentence-final, so it is often pronounced with a fuller dipping contour in careful speech, though in natural speech tones may be somewhat reduced depending on rhythm.

For a learner, the main practical point is: pronounce it roughly as

wǒ gēnběn bù dǒng

Sometimes, but I don’t understand at all is usually better.

Because 懂 is about understanding, translating it as know can be misleading in English.

Depending on context, possible translations are:

• I don’t understand at all.
• I simply don’t understand.
• I have no idea.

If the context is a skill or field of knowledge, English might use know more naturally:

• 我根本不懂电脑。 = I really know nothing about computers.

So the best translation depends on context.

Yes, if the subject is clear from context.

Chinese often drops subjects when they are understood. So in conversation, someone may simply say:

• 根本不懂。

That can still mean I don’t understand at all, you don’t understand at all, or he doesn’t understand at all, depending on context.

Including 我 makes the subject explicit.

Yes, it is natural and common.

Native speakers use this kind of structure often, especially when expressing strong lack of understanding:

• 我根本不懂。
• 我根本听不懂。 = I can’t understand it at all when I hear it.
• 我根本看不懂。 = I can’t understand it at all when I read it.

In real speech, people also often extend it:

• 我根本不懂你在说什么。
• 这个我根本不懂。

So the sentence is short, natural, and useful.

Both can relate to understanding, but they are not always identical.

• 懂 often means understand, comprehend, or know about
• 明白 often means to understand clearly or to become clear

So:

• 我不懂。 = I don’t understand / I’m not knowledgeable about it.
• 我不明白。 = I don’t understand / It’s not clear to me.

In many situations they are close:

• 我根本不懂。
• 我根本不明白。

But 不明白 often feels more like I don’t see what you mean or it’s unclear to me, while 不懂 can also suggest a deeper lack of familiarity or knowledge.

By itself, it is not rude, but it can sound blunt.

Tone of voice and context matter a lot.

• Said about yourself in a humble or frustrated way: usually fine
• Said in response to someone else: can sound abrupt
• Said about another person: can sound critical

For example:

• 我根本不懂这个。 = usually harmless
• 你根本不懂。 = You simply don’t understand — this can sound confrontational

So the phrase is natural, but because 根本 is emphatic, it can sound strong.