Teprve teď rozumím této myšlence.

Here teprve means only / not until.

So Teprve teď rozumím této myšlence is not just Now I understand this idea, but more specifically:

• Only now I understand this idea
• Not until now do I understand this idea
• I’m only now understanding this idea

It adds the feeling that the understanding came late or later than expected.

Teď just means now.

• Teď rozumím této myšlence. = Now I understand this idea.
• Teprve teď rozumím této myšlence. = Only now do I understand this idea.

So teprve adds emphasis: the speaker did not understand it before, and the understanding is happening only at this moment.

In Czech, the subject pronoun is often omitted because the verb ending already shows the person.

• rozumím = I understand

So já is not necessary.

If you say Já teprve teď rozumím této myšlence, it sounds more emphatic, as if you are contrasting yourself with someone else or stressing I.

Because the verb rozumět takes the dative case, not the accusative.

This is very important for learners, because English understand takes a direct object, but Czech rozumět works differently:

• rozumět komu? čemu? = to understand someone/something
  literally, it behaves like a verb that governs the dative

So:

• této myšlence = dative singular = to this thought/idea

Not:

• tuto myšlenku = accusative singular

It is the dative singular form of:

• tato = this (feminine nominative)
• myšlenka = thought / idea

1) tato → této

This is the feminine singular dative form of tento / tato / toto.

2) myšlenka → myšlence

myšlenka is a feminine noun ending in -a.
In the dative singular, it becomes -e, and the k changes to c:

• myšlenka
• myšlence

That consonant change is normal in Czech declension.

Because Czech often changes certain consonants before endings like -e.

Here:

• k → c

So:

• myšlenka → myšlence

This happens in many nouns and is a regular part of Czech declension. It may feel strange at first if you are used to English, where nouns usually do not change internally like this.

Yes, often you could.

• Rozumím této myšlence.
• Chápu tu myšlenku.

Both can mean I understand this idea, but there are some differences:

rozumět

• often focuses on understanding the meaning/content
• takes the dative
• also used in expressions like rozumím česky or rozumím tomu

chápat

• often means to grasp / comprehend
• usually takes the accusative
• can sound a bit more like mentally grasping the whole point

So both are possible in many contexts, but the grammar changes:

• rozumím této myšlence
• chápu tuto / tu myšlenku

Yes, it is correct and natural, but it sounds a bit more formal or written than some everyday alternatives.

In spoken Czech, people often use forms like:

• téhle myšlence
• té myšlence
• tomu nápadu depending on meaning and style

So této myšlence is perfectly good standard Czech, but in casual conversation a speaker might choose a less formal-sounding phrase.

Yes. Czech word order is more flexible than English because the endings show grammatical relationships.

The original sentence:

• Teprve teď rozumím této myšlence.

Possible variants:

• Této myšlence rozumím teprve teď.
• Rozumím této myšlence teprve teď.

These all mean roughly the same thing, but the emphasis changes.

For example:

• Teprve teď... puts focus on only now
• Této myšlence... puts focus on this idea
• Rozumím... first can sound more neutral in some contexts

So the original order is natural, but not the only possible one.

They are very close, and in many contexts both can mean only now.

• Teprve teď rozumím této myšlence.
• Až teď rozumím této myšlence.

Both are possible, but there can be a slight difference in feel:

• teprve teď = neutral, standard only now
• až teď = often a little more emotional or conversational, sometimes emphasizing surprise or delay

So if you are learning, teprve teď is a very safe and useful form.

The negative is:

• Teprve teď této myšlence nerozumím. = probably not the intended meaning
• Této myšlence stále nerozumím. = I still don’t understand this idea
• Doteď jsem této myšlence nerozuměl / nerozuměla. = Until now, I didn’t understand this idea

The direct negative of rozumím is nerozumím.

But in practice, if you want the opposite idea, Czech usually phrases it differently depending on the exact meaning:

• I still don’t understand this idea
• Until now I didn’t understand this idea

So the grammar is simple, but the most natural sentence may not be a word-for-word negative mirror of the original.