Ten plan to szaleństwo.

Because plan is a masculine singular noun in Polish.

The demonstrative ten means this for a masculine singular noun:

• ten = masculine
• ta = feminine
• to = neuter

So:

• ten plan = this plan
• ta idea = this idea
• to szaleństwo = this madness / it is madness

A native English speaker often expects one word for this, but in Polish it changes to match the noun’s gender.

Here, to works like is in an identifying sentence.

So Ten plan to szaleństwo literally works like:

• This plan = madness

In Polish, to is very commonly used in sentences where one thing is identified as another thing:

• To mój brat. = This is my brother.
• Warszawa to stolica Polski. = Warsaw is the capital of Poland.
• Ten plan to szaleństwo. = This plan is madness.

So this to is not the English to from to go or to eat. It has a completely different job.

Because in this kind of sentence, Polish often uses to instead of jest in the present tense.

So:

• Ten plan to szaleństwo is natural.
• You do not normally say Ten plan to jest szaleństwo.

In many present-tense identification sentences, to is enough by itself:

• To problem.
• Jan to mój nauczyciel.
• Ten plan to szaleństwo.

This is one of the first things English speakers notice, because English usually requires is, while Polish often does not in this pattern.

Because szaleństwo means madness/insanity, which is a noun.

The sentence is not saying:

• This plan is crazy

It is saying:

• This plan is madness

That is a slightly different idea and tone.

Compare:

• Ten plan to szaleństwo. = This plan is madness.
• Ten plan jest szalony. = This plan is crazy.

So:

• szaleństwo = noun
• szalony = adjective

English can use either structure very naturally, but Polish uses different grammar depending on whether you choose a noun or an adjective.

Here szaleństwo stays in the nominative form.

That is because in sentences with to like this, both parts are typically in the nominative:

• Ten plan — nominative
• szaleństwo — nominative

So:

• Ten plan to szaleństwo.

This is different from another common Polish pattern with jest, where a predicate noun is often in the instrumental:

• Ten plan jest szaleństwem.

Both can be translated as This plan is madness, but the grammar is different:

• with to → nominative
• with jest
  • predicate noun → often instrumental

Yes, you can.

Both are correct:

• Ten plan to szaleństwo.
• Ten plan jest szaleństwem.

But they use different structures.

Ten plan to szaleństwo

• uses to
• sounds like a direct identification: This plan is madness

Ten plan jest szaleństwem

• uses jest
• uses the instrumental form szaleństwem
• also means This plan is madness

For a learner, the important thing is not to mix the two patterns:

• Ten plan to szaleństwo ✅
• Ten plan jest szaleństwem ✅
• Ten plan to szaleństwem ❌
• Ten plan jest szaleństwo ❌

Yes, it is natural and idiomatic.

It sounds like a strong judgment:

• This plan is madness.
• This plan is insane.

It is a normal way to react to an idea that seems reckless, absurd, or dangerous.

A very common shorter version is also:

• To szaleństwo! = It’s madness!

So if the context already makes the plan clear, a speaker might simply say To szaleństwo!

The normal order here is:

• Ten plan to szaleństwo.

That order clearly presents the topic first (this plan) and then identifies it (madness).

Polish word order is flexible, but not every rearrangement sounds equally natural. This version is the most straightforward one.

For emphasis, speakers may move things around in some contexts, but for a learner, Ten plan to szaleństwo is the best model to follow.

A rough guide is:

sha-LEN-stvo

More carefully:

• sza sounds like sha
• leń has a soft ń, something like the ny sound in canyon, but shorter
• stwo sounds roughly like stvo

The stress is on the second-to-last syllable:

• sza-LEŃ-stwo

A more phonetic approximation is:

• sha-LEN-stfaw / sha-LEN-stvo

The exact Polish sounds are a bit different from English, especially sz, ń, and the consonant cluster in -stwo.

It sounds strong and expressive.

Ten plan to szaleństwo is not a neutral description. It usually means the speaker thinks the plan is:

• extremely bad
• reckless
• absurd
• dangerous

So it carries emotion, not just information.

In English, the tone is similar to:

• This plan is madness.
• This plan is insane.
• This is a crazy plan.

Because plan is the subject-like part of the sentence and is in the basic dictionary form, the nominative singular.

You are saying:

• this plan = madness

So:

• ten matches plan
• plan stays nominative

If the sentence were in another grammatical context, the form could change:

• Nie rozumiem tego planu. = I don’t understand this plan.
• Myślę o tym planie. = I’m thinking about this plan.

But in Ten plan to szaleństwo, it stays as plan.

No, not if you mean this plan.

That would be wrong because plan is masculine, so the demonstrative must be ten:

• ten plan ✅
• to plan ❌

Remember:

• ten for masculine singular nouns
• ta for feminine singular nouns
• to for neuter singular nouns

So:

• ten plan
• ta książka
• to okno