Společný národní plán je jednoduchý.

Because they are adjectives agreeing with plán.

In Czech, adjectives change their endings to match the noun’s:

• gender
• number
• case

Here, plán is:

• masculine inanimate
• singular
• nominative (the subject of the sentence)

So the adjectives take the matching nominative singular masculine inanimate ending -ý:

• společný = common/shared
• národní = national
• jednoduchý = simple

So all three are in the form that matches plán.

Plán is masculine inanimate.

You often learn noun gender as part of the vocabulary item, but there are useful clues:

• many masculine nouns end in a consonant
• plán ends in a consonant, so masculine is a good first guess

It is specifically inanimate, because it refers to a thing, not a living male being.

This matters because adjectives and some other words must agree with it.

Czech allows multiple adjectives before a noun, just like English:

• společný národní plán = common national plan

Both adjectives describe plán.

A learner might wonder whether one adjective is more closely tied to the noun than the other. In practice, both simply modify plán, and this word order is normal.

Je is the 3rd person singular form of the verb být = to be.

So:

• plán je jednoduchý = the plan is simple

Because plán is singular, Czech uses je:

• já jsem = I am
• ty jsi = you are
• on/ona/ono je = he/she/it is

Here the subject is společný národní plán, so je is the correct form.

Because it is a predicate adjective after je, and in Czech it still agrees with the subject.

In English, adjectives do not change:

• the plan is simple
• the plans are simple

But in Czech, the adjective changes to match the subject:

• Plán je jednoduchý.
• Plány jsou jednoduché.

So jednoduchý matches plán in gender, number, and case.

The main noun phrase společný národní plán is in the nominative, because it is the subject of the sentence.

Structure:

• Společný národní plán = subject, nominative
• je = is
• jednoduchý = predicate adjective agreeing with the subject

So this is a basic nominative sentence pattern: X je Y = X is Y

Yes, Czech word order is more flexible than English, but the neutral order here is very natural:

• Společný národní plán je jednoduchý.

You might also see:

• Jednoduchý je společný národní plán.

That version gives a different emphasis, almost like:

• It is the common national plan that is simple or
• The common national plan is the simple one

For learners, the original order is the safest and most neutral.

It can mean both, depending on context.

Společný often means:

• shared
• joint
• common

So společný národní plán could suggest a plan that is shared jointly by a nation or by multiple parties at a national level, depending on the broader context.

This is one of those words whose exact English translation depends on the situation.

Usually jednoduchý means simple in the sense of not complicated.

It can sometimes overlap with easy, but Czech often distinguishes:

• jednoduchý = simple, straightforward
• snadný / lehký = easy

So in this sentence, jednoduchý most naturally means simple.

A rough pronunciation guide for an English speaker is:

• Společný ≈ SPO-lech-nee
• národní ≈ NAA-rod-nee
• plán ≈ plaan
• je ≈ ye
• jednoduchý ≈ YE-dno-doo-khee

A few important sounds:

• č = like ch in church
• á, í, ý are long vowels
• j is pronounced like English y
• ch is a harsh sound, like German Bach or Scottish loch

So jednoduchý is not pronounced with English j at the start.

Czech has no articles like English a/an/the.

So:

• plán can mean a plan or the plan

Which one is meant depends on context.

That is why Společný národní plán je jednoduchý could be translated as:

• The common national plan is simple or
• A common national plan is simple

Usually the surrounding context tells you which is intended.

It is grammatically normal and neutral.

The sentence sounds a bit like written or official language because of the vocabulary:

• národní plán = national plan
• společný = common/joint/shared

So it feels like something you might read in a policy document, report, or formal discussion, but the grammar itself is completely standard.