Samozřejmě rozumím plánu.

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

• Base form: plán = plan
• Dative singular: plánu

So rozumím plánu literally means I understand to the plan, but in natural English we just say I understand the plan.

This is something English speakers often have to get used to: Czech verbs often control a specific case, and it may not match what English does.

Czech often leaves out subject pronouns when they are already clear from the verb ending.

Here, rozumím means I understand. The ending -ím shows 1st person singular, so já is unnecessary.

You could say Já samozřejmě rozumím plánu, but that adds emphasis, something like I do understand the plan, of course.

So the version without já is the normal, neutral one.

Rozumím is the 1st person singular present tense of the infinitive rozumět.

So:

• rozumět = to understand
• rozumím = I understand

This verb is very common, and it is also used for understanding languages, for example:

• Rozumím česky. = I understand Czech.

Yes, in standard Czech, rozumět takes the dative.

Examples:

• Rozumím plánu. = I understand the plan.
• Rozumím učiteli. = I understand the teacher.
• Rozumím češtině. = I understand Czech.

This is an important pattern to memorize as a whole: rozumět + dative.

Samozřejmě means of course, naturally, or obviously, depending on context.

In this sentence, it most naturally means of course:

• Samozřejmě rozumím plánu. = Of course I understand the plan.

It adds the speaker’s attitude: the speaker presents their understanding as something expected or obvious.

Not here.

Samozřejmě rozumím plánu. is a normal sentence without a comma.

A comma may appear when samozřejmě is used more like a parenthetical comment, but in this simple sentence it is usually written without one.

So the form you have is perfectly standard.

Yes. Czech word order is flexible, and changing it changes the emphasis more than the basic meaning.

For example:

• Samozřejmě rozumím plánu. = neutral, with of course up front
• Rozumím plánu samozřejmě. = possible, but less natural in many contexts
• Plánu samozřejmě rozumím. = emphasizes the plan

The original sentence is a very natural choice.

They overlap, but they are not always identical.

• rozumět often means to understand, to follow, or to know the meaning of
• chápat often means to grasp, to comprehend, or to understand intellectually

In many situations either can work, but rozumět is the normal verb in a sentence like this, especially with the structure rozumět + dative.

So Rozumím plánu sounds very natural.

A helpful rough guide is:

sa-moz-zhrey-mnye ro-zu-meem plaa-noo

A few important points:

• Czech stress is usually on the first syllable of each word:
  • SAmozřejmě
  • ROzumím
  • PLÁnu
• ř in samozřejmě is a special Czech sound with no exact English equivalent, so learners often need practice with it.
• á is a long vowel, so plánu has a longer a sound than planu would.

You do not need perfect pronunciation immediately, but it helps to notice vowel length and first-syllable stress early.