wǒ wánquán dǒng le.

Here, 了 marks a new situation or change of state.

So 懂 means understand, but 懂了 means something like:

• now I understand
• I've got it
• I understand now

It does not simply mean past tense. It shows that your state has changed from not understanding before to understanding now.

So:

• 我懂。 = I understand / I know it
• 我懂了。 = I understand now / I get it now

With 完全, the sentence becomes stronger:

• 我完全懂了。 = I completely understand now

No. That is a very common misunderstanding for English speakers.

In this sentence, 了 does not mean a simple English-style past tense like understood. Instead, it marks that something has changed or has become true now.

Compare:

• 我懂。 = I understand
• 我懂了。 = I understand now / I've come to understand

So if a teacher explains something and you suddenly get it, 我懂了 is natural.

If you treat 了 as just past tense, you may miss the real idea: the speaker has reached understanding.

Chinese word order here is:

• 我 = subject
• 完全 = adverb
• 懂 = verb
• 了 = sentence-final/change-of-state particle attached to the verb phrase

So the pattern is:

Subject + adverb + verb + 了

That is why 完全 comes before 懂. It modifies the verb, like completely in English:

• 我完全懂了。 = I completely understand now.

Putting 完全 after the verb would usually sound wrong in this sentence.

In this sentence, 完全 is functioning as an adverb, meaning:

• completely
• totally

It modifies 懂:

• 完全懂 = understand completely

Although 完全 can also be part of adjective-like expressions in other contexts, here it is best understood as an adverb modifying the verb.

Examples:

• 我完全同意。 = I completely agree.
• 他完全不知道。 = He totally doesn't know.

Good question. 懂 often does take an object, such as:

• 我懂这个问题。 = I understand this question.
• 我懂你的意思了。 = I understand what you mean now.

But in conversation, the object is often left out when it is obvious from context.

So if someone has just explained something, saying:

• 我完全懂了。

naturally means:

• I completely understand it now.

The thing understood is understood from the situation, so Chinese does not need to say it explicitly.

我懂了 means:

• I understand now
• I get it

我完全懂了 adds extra emphasis:

• I completely understand now
• I fully get it

So 完全 makes the statement stronger.

In real life, this can sometimes sound very definite. If you are not fully sure, native speakers might use softer versions like:

• 我懂一点了。 = I understand a bit now.
• 我大概懂了。 = I mostly get it now.
• 我差不多懂了。 = I more or less understand now.

Yes, 我全懂了 is also possible in conversation.

The difference is roughly:

• 我完全懂了 = I completely understand now
• 我全懂了 = I understand all of it now / I get everything now

So:

• 完全 emphasizes degree: completely
• 全 often emphasizes the whole thing: all of it

In many contexts they are close, but they are not always exactly the same.

Often, yes.

• 我完全懂了。
• 我完全明白了。

Both can mean I completely understand now.

A rough difference:

• 懂 often feels like understand / know how / grasp
• 明白 often feels like understand clearly / realize

In many everyday situations, both work. But 懂 is especially common when talking about understanding content, language, skills, or explanations.

Examples:

• 我听懂了。 = I understood what I heard.
• 我明白了。 = I understand now / I see.

Yes, it can.

我完全懂了 is very strong because it says you understand completely. If you later ask basic follow-up questions, it may sound inconsistent.

So learners often want to choose carefully:

• 我懂了。 = natural and common
• 我差不多懂了。 = I mostly understand now
• 我有点懂了。 = I'm starting to understand
• 我还没完全懂。 = I still don't completely understand

If you truly mean I fully understand now, then 我完全懂了 is fine.

No, that sounds unnatural.

English speakers often want to use 很 for many kinds of emphasis, but 很 does not work well here.

Use:

• 我完全懂了。
• 我都懂了。
• 我真的懂了。

Not:

• 我很懂了。

Why? Because 很 usually modifies adjectives or certain stative verbs in ways that do not fit naturally with 懂了 in this meaning.

The dictionary tones are:

• 懂 = dǒng (3rd tone)
• 了 = le (neutral tone here)

So you say:

• dǒng le

The tone on 懂 stays a 3rd tone, and 了 is light and neutral.

For the full sentence:

• 我 = wǒ (3rd tone)
• 完全 = wánquán (2nd tone + 2nd tone)
• 懂 = dǒng (3rd tone)
• 了 = le (neutral)

So:

wǒ wánquán dǒng le

In natural speech, the sentence flows smoothly, with 了 unstressed.

Usually after an explanation, demonstration, or clarification.

For example:

• A teacher explains a grammar point.
• A friend explains the rules of a game.
• Someone repeats something more clearly.
• You suddenly understand a problem.

Then you might say:

• 哦，我完全懂了。 = Oh, I completely understand now.

It is especially natural when there is a contrast with an earlier lack of understanding:

• 刚才我不懂，现在我完全懂了。
• I didn't understand just now, but now I completely understand.