Tämä ajatus on hullu.

Tämä means this. In this sentence, it points to a specific idea or thought: tämä ajatus = this idea.

It is in the basic singular form because the whole subject of the sentence is singular and in the nominative.

Finnish does not have articles like English a and the.

So ajatus can mean an idea, the idea, or just idea, depending on context. In this sentence, tämä already makes it definite, so tämä ajatus clearly means this idea.

Because ajatus is the subject of the sentence, and subjects are very often in the nominative, which is the dictionary form.

So here:

• tämä ajatus = the subject
• on = the verb
• hullu = what is being said about the subject

On is the 3rd person singular present form of olla, which means to be.

Since tämä ajatus is singular, Finnish uses on:

• minä olen = I am
• sinä olet = you are
• tämä ajatus on = this idea is

So on is simply the correct form of to be for a singular subject.

Here hullu is a predicate adjective, meaning it comes after on and describes the subject.

With a singular nominative subject like tämä ajatus, the adjective is also in the basic singular form:

• Ajatus on hullu = The idea is crazy

This is different from an adjective used directly before a noun, but in this case the form happens to be the same:

• hullu ajatus = a crazy idea
• ajatus on hullu = the idea is crazy

Ajatus can mean both thought and idea, depending on context.

That is very common in Finnish. If the English translation shown to the learner is idea, then ajatus is being used in that sense here. In another context, it could just as easily mean thought.

Yes, it is understandable and grammatically correct.

However, hullu is quite a strong word. Literally, it can mean crazy, mad, or even insane. When used about an idea, it usually sounds metaphorical, like this idea is crazy or this is a wild idea.

Depending on context, Finnish speakers might also choose other adjectives, such as:

• outo = strange
• älytön = absurd
• hurja = wild, intense

But hullu is still possible and clear.

Yes, but the structure is different.

• hullu ajatus = a crazy idea
• ajatus on hullu = the idea is crazy

So the first is a noun phrase, and the second is a full sentence.

English works the same way:

• a crazy idea
• the idea is crazy

The basic, neutral word order here is:

• Tämä ajatus
  • on
    • hullu

That is the safest and most standard order for learners.

Finnish word order is more flexible than English, so other orders are possible for emphasis, but they can sound more marked or expressive. If you are just learning, Tämä ajatus on hullu is the best version to use.

You would say:

Tämä ajatus ei ole hullu.

Finnish negation uses a negative verb:

• ei = negative auxiliary
• ole = the main verb form used after it

So:

• on = is
• ei ole = is not

A direct plural version would begin with Nämä ajatukset = these ideas.

So the sentence would be:

Nämä ajatukset ovat hulluja.

Notice the changes:

• tämä → nämä = this → these
• ajatus → ajatukset = idea → ideas
• on → ovat = is → are
• hullu → hulluja = crazy, in the plural predicate form

This is a good example of how several words change when the subject becomes plural.