Cartea aceasta este interesantă.

In Romanian, the definite article is usually attached to the end of the noun instead of being a separate word like English the.

So:

• carte = book
• cartea = the book

The -a here is the feminine singular definite article.

Romanian often places the demonstrative after the noun, especially in everyday usage.

So:

• cartea aceasta = this book

This is very common and natural in Romanian.

You may also see:

• această carte

That also means this book, but the post-noun version cartea aceasta is extremely common and often feels more neutral or conversational.

Because carte is a feminine noun.

Romanian words that refer back to nouns must agree with the noun in gender and number.

• masculine singular: acesta = this
• feminine singular: aceasta = this

Since carte is feminine, the correct form is aceasta.

Because the adjective must agree with the noun.

Since cartea is:

• feminine
• singular

the adjective must also be feminine singular:

• masculine singular: interesant
• feminine singular: interesantă

So:

• un film interesant = an interesting film
• o carte interesantă = an interesting book

Este is the 3rd person singular of a fi = to be.

So este means is.

Yes, in everyday Romanian, people very often use e instead of este:

• Cartea aceasta este interesantă.
• Cartea aceasta e interesantă.

Both are correct. E is shorter and more common in speech.

The standard word order here is very natural:

• Cartea aceasta este interesantă.

Romanian does allow some flexibility, but not every change sounds equally natural.

For example:

• Această carte este interesantă. — also correct
• Interesantă este cartea aceasta. — possible, but more marked or emphatic

So for a learner, the safest choices are:

• Cartea aceasta este interesantă.
• Această carte este interesantă.

Because Romanian usually expresses the by adding it to the noun.

Compare:

• English: the book
• Romanian: cartea

This is one of the big differences between Romanian and English. The article is called an enclitic definite article because it attaches to the end of the noun.

You usually learn the gender of Romanian nouns together with the noun itself. A very common way is to memorize the indefinite form with the article:

• o carte = a book

Since it takes o, it is feminine.

Then other words must match that gender:

• cartea
• aceasta
• interesantă

All of those feminine forms help confirm the noun’s gender.

A rough pronunciation is:

• aceasta ≈ a-CHAS-ta

The stress is usually on the second syllable: ceas.

A few helpful points:

• c before e sounds like ch in chair
• ea is pronounced together as a vowel sequence
• the final a is a normal a, not ă

So aceasta is different from forms containing ă.

The letter ă is a very common Romanian vowel. It is a short central vowel, somewhat like the a in English sofa in unstressed position, but you should not rely on English too much.

So:

• interesantă ends with -tă, not -ta

A rough guide:

• interesantă ≈ in-te-re-SAN-tuh

But the final sound is specifically Romanian ă, not a full English uh.

Yes.

• Cartea este interesantă. = The book is interesting.
• Cartea aceasta este interesantă. = This book is interesting.

So aceasta adds the idea of this. Without it, the sentence is more general and just means the book.

It can, but that would be a different structure.

Your sentence is:

• Cartea aceasta este interesantă.
• literally: This book is interesting

If you want this interesting book, where interesting directly describes the noun inside the noun phrase, you would say something like:

• cartea aceasta interesantă in a very specific context, though this exact structure is less common by itself
• more naturally in many cases: această carte interesantă

So in your sentence, interesantă is part of the predicate after is, not just an adjective inside the noun phrase.

Yes.

In Cartea aceasta este interesantă, cartea aceasta is the subject of the sentence, so it is in the nominative.

For many feminine singular nouns, the nominative and accusative forms look the same, so you do not see a special change here. Romanian cases matter, but in this sentence nothing unusual is happening: it is a straightforward subject + verb + adjective sentence.