Plan jest kompletnie gotowy.

Because plan is the subject of the sentence, so it appears in the nominative case.

• plan = nominative singular
• It is a masculine inanimate noun.

In this sentence:

• Plan = the thing being talked about
• jest gotowy = says something about that subject

So Polish works very much like English here: the subject stays in its normal form.

Jest is the 3rd person singular form of być (to be).

It is used because the sentence has the structure:

• subject + to be + adjective

So:

• Plan = subject
• jest = is
• gotowy = adjective describing the subject

In Polish, present-tense być is normally not omitted in sentences like this. So:

• Plan jest gotowy = correct
• Plan gotowy = possible only in special headline-style or very informal elliptical speech, not the normal full sentence

Because adjectives in Polish agree with the noun they describe in:

• gender
• number
• case

Here, plan is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine singular nominative → gotowy

Compare:

• Plan jest gotowy — masculine singular
• Lista jest gotowa — feminine singular
• Zadanie jest gotowe — neuter singular
• Plany są gotowe — plural

Kompletnie is an adverb modifying gotowy. It intensifies it, like completely or totally in English.

So it answers the question:

• How ready? → completely ready

Grammatically:

• gotowy = adjective
• kompletnie = adverb modifying that adjective

This is similar to English:

• completely ready
• totally finished
• fully prepared

It is grammatical, but it may sound a bit emphatic or stylistically marked.

Polish speakers often use alternatives such as:

• Plan jest całkowicie gotowy.
• Plan jest już gotowy.
• Plan jest w pełni gotowy. (less common, more formal/technical depending on context)

A useful nuance:

• kompletnie often sounds strong, sometimes even a bit colloquial
• It is especially common in negative phrases like kompletnie bez sensu or kompletnie nieprzygotowany

So Plan jest kompletnie gotowy is understandable and correct, but in everyday speech many speakers might more naturally say Plan jest już gotowy if they simply mean the plan is finished and ready.

Yes, Polish word order is more flexible than English, but not every order sounds equally natural.

The most neutral version is:

• Plan jest kompletnie gotowy.

Other possible orders include:

• Plan jest gotowy kompletnie. — possible, but less natural
• Kompletnie gotowy jest plan. — marked, used for emphasis or style
• Gotowy jest plan. — possible, more literary or contrastive

In normal conversation, the original order is the safest and most natural.

Polish has no articles, so there is no direct equivalent of the or a/an.

So:

• plan can mean a plan or the plan

You understand which one is meant from context.

This is very normal in Polish:

• Mam książkę. = I have a book / the book
• Plan jest gotowy. = The plan is ready / A plan is ready

Sometimes yes, but the meaning shifts slightly.

• gotowy = ready
• przygotowany = prepared

So:

• Plan jest gotowy focuses on readiness or completion
• Plan jest przygotowany focuses more on the fact that someone has prepared it

In many contexts they may overlap, but they are not always identical.

For example:

• Plan jest gotowy = the plan is ready to use/present
• Plan jest przygotowany = the plan has been prepared

If you just want the normal idea of ready, gotowy is the best choice.

Yes, absolutely.

• Plan jest gotowy. = a very normal, natural sentence
• Plan jest kompletnie gotowy. = stronger emphasis

Since gotowy already means ready, adding kompletnie is only for extra force. In many situations, the simpler sentence sounds more natural.

Yes. Gotowy is the standard everyday adjective form here.

There is also gotów, but it is different in feel and usage. It can sound:

• more literary
• more fixed-expression-like
• sometimes closer to willing, prepared, or even likely/about to, depending on context

For a simple sentence like this, use:

• Plan jest gotowy.

That is the normal modern choice.

A careful approximate pronunciation is:

• Plan jest kompletnie gotowy
• roughly: plan yest kom-PLET-nye go-TO-vih

A few helpful notes:

• jest sounds like yest
• ie in kompletnie sounds roughly like nye at the end
• w in gotowy is pronounced like English v
• Polish stress is usually on the second-to-last syllable:
  • kom-plet-nie
  • go-to-wy

So the main stresses are:

• kom-PLET-nie
• go-TO-wy

Here is the breakdown:

• Plan — noun
• jest — verb, 3rd person singular of być
• kompletnie — adverb
• gotowy — adjective

So the structure is:

• noun + verb + adverb + adjective

Or more functionally:

• subject + copula + intensifier + predicate adjective

That is a very common Polish sentence pattern.