Tämä kirja on hyvä lähde.

Word by word:

• tämä = this
• kirja = book
• on = is
• hyvä = good
• lähde = source

So the structure is very straightforward: This book is a good source.

Finnish does not normally use articles like English a, an, or the.

So:

• kirja can mean a book or the book, depending on context
• lähde can mean a source or the source, depending on context

In this sentence, English naturally translates it as This book is a good source, but Finnish does not need separate article words to say that.

Tämä is a demonstrative word meaning this.

In Tämä kirja, it points to a specific book: this book.

A useful comparison:

• kirja = book / a book / the book
• tämä kirja = this book

So tämä makes the noun more specific.

Because after the verb olla (to be), Finnish often uses the predicate noun and predicate adjective in the nominative basic form, especially in simple sentences like this one.

So:

• hyvä = nominative singular
• lähde = nominative singular

That is why you get:

• Tämä kirja on hyvä lähde.

and not something like:

• Tämä kirja on hyvää lähdettä. in this context

The sentence is simply identifying what kind of thing the book is: a good source.

Because in Finnish, adjectives usually come before the noun they describe, just like in English.

So:

• hyvä lähde = good source
• iso talo = big house
• uusi kirja = new book

That makes hyvä lähde the normal order.

Yes. In Finnish, adjectives agree with the nouns they modify in number and usually case.

Here both are singular nominative:

• hyvä = singular nominative
• lähde = singular nominative

So they match.

For example, in the plural you would get:

• hyvät lähteet = good sources

The adjective changes together with the noun.

On is the third-person singular form of olla (to be).

Because the subject is tämä kirja (this book), which is singular, the verb is singular too:

• kirja on = the book is
• tämä kirja on = this book is

If the subject were plural, the verb would change:

• Nämä kirjat ovat hyviä lähteitä. = These books are good sources.

Tämä kirja on hyvä lähde is the most neutral and natural word order.

Finnish word order is fairly flexible, but changing it usually changes the focus or emphasis.

For example:

• Tämä kirja on hyvä lähde. = neutral statement
• Hyvä lähde on tämä kirja. = more marked; emphasizes good source
• On tämä kirja hyvä lähde. = possible in some contexts, but not the basic neutral form

So for learners, Tämä kirja on hyvä lähde is the best default pattern.

Yes. Lähde can also be the imperative form of the verb lähteä, meaning leave! or go!

But in this sentence, lähde is clearly a noun meaning source.

Why it is clearly a noun here:

• it comes after hyvä (good), which describes nouns
• it follows on (is) in a normal X is Y sentence
• the whole phrase hyvä lähde means a good source

So here there is no ambiguity in practice.

A simple learner-friendly guide:

• Tämä ≈ TEH-mae
• kirja ≈ KEER-ya
• on ≈ on
• hyvä ≈ HUU-va or HUU-væ depending on how closely you want the vowels
• lähde ≈ LAEH-hdeh

A few important pronunciation notes:

• Finnish stress is usually on the first syllable of each word.
• ä is not the same as English a. It is more like the vowel in British English cat, but keep it short and clear.
• y in hyvä is a front rounded vowel that English does not really have. A rough approximation is okay at first.
• Finnish pronunciation is usually very regular, so once you know the sounds, the spelling is dependable.

You would say:

• Nämä kirjat ovat hyviä lähteitä.
• These books are good sources.

Notice the changes:

• tämä → nämä = this → these
• kirja → kirjat = book → books
• on → ovat = is → are
• hyvä → hyviä
• lähde → lähteitä

The last two change because plural predicate expressions in Finnish often appear in the partitive plural in sentences like this.

Because Finnish is grouping the words in a specific way:

• Tämä kirja = the subject, this book
• on = the verb, is
• hyvä lähde = the predicate, a good source

So the meaning is:

• [This book] [is] [a good source].

If you said hyvä kirja, that would mean a good book, which is a different idea.

Compare:

• Tämä kirja on hyvä. = This book is good.
• Tämä kirja on hyvä lähde. = This book is a good source.

So hyvä is describing lähde, not kirja.