Questa idea è assurda.

Because idea is a feminine singular noun in Italian, and questa has to agree with it.

The demonstrative questo changes form depending on gender and number:

• questo = masculine singular
• questa = feminine singular
• questi = masculine plural
• queste = feminine plural

So:

• questo libro = this book
• questa idea = this idea

Even though English this never changes, Italian requires agreement.

For the same reason: adjectives in Italian usually agree with the noun they describe.

Here, assurda describes idea, which is feminine singular, so the adjective must also be feminine singular:

• masculine singular: assurdo
• feminine singular: assurda
• masculine plural: assurdi
• feminine plural: assurde

So:

• Questo piano è assurdo. = This plan is absurd.
• Questa idea è assurda. = This idea is absurd.

Yes, idea is definitely feminine: un'idea, questa idea, idea assurda.

Many Italian nouns ending in -a are feminine, and idea follows that pattern.

A useful thing to notice is that although the singular is feminine, the plural is idee:

• l'idea
• le idee

So if you made the sentence plural, you would get:

• Queste idee sono assurde.

Because questa already functions as a determiner, like English this.

In English, you say:

• this idea

not

• the this idea

Italian works similarly here:

• questa idea

not normally

• la questa idea

So questa takes the place that an article like la would usually occupy.

The accent distinguishes the verb è (is) from e (and).

• è = is
• e = and

This is very important in writing:

• Questa idea è assurda. = This idea is absurd.
• pane e burro = bread and butter

The accented è is the 3rd person singular of essere.

Yes. Quest'idea è assurda is also correct.

Before a vowel, questa can be shortened to quest':

• questa idea
• quest'idea

Both are understood, but quest'idea often sounds more natural and smoother because idea begins with a vowel.

This shortening is called elision.

Here it is a demonstrative adjective because it comes directly before a noun: questa idea.

Compare:

• Questa idea è assurda. → questa modifies idea, so it is an adjective.
• Questa è assurda. → questa stands alone, so it is a pronoun.

That distinction is useful because Italian demonstratives can do both jobs.

Yes, this is the most normal and neutral word order.

It follows a very common pattern:

• subject: Questa idea
• verb: è
• predicate adjective: assurda

So it works much like English This idea is absurd.

Italian can change word order for emphasis, but the basic version here is the standard one.

Yes, but it is not exactly the same structure.

• Questa idea è assurda. = This idea is absurd.
• Questa è un'idea assurda. = This is an absurd idea.

The second sentence uses questa as a pronoun (this) and then introduces un'idea assurda. The meaning is very close, but the grammar is different.

It is pronounced roughly ahs-SOOR-dah.

A few helpful details:

• the ss is pronounced as a clear double consonant
• the stress is on sur
• the final -a is pronounced clearly

So not like an English reduced ending, but a full vowel: as-SUR-da.

Everything that agrees with the noun would change:

• Queste idee sono assurde. = These ideas are absurd.

Changes:

• questa → queste
• idea → idee
• è → sono
• assurda → assurde

This is a good example of how Italian agreement works across the whole sentence.