En usko tätä uutista.

Because Finnish often leaves out the subject pronoun when the verb already shows the person.

Here, en is the 1st person singular negative form, so it already tells you the subject is I. That makes En usko... completely natural.

You can add minä if you want emphasis:

• Minä en usko tätä uutista. = I don’t believe this news.

That sounds a bit more emphatic or contrastive than the version without minä.

Finnish does not usually use a separate unchanging word like English not. Instead, it uses a negative verb that changes according to the subject.

Present tense negative forms:

• en = I do not
• et = you do not
• ei = he/she/it does not
• emme = we do not
• ette = you all do not
• eivät = they do not

So:

• en usko = I do not believe
• et usko = you do not believe
• ei usko = he/she does not believe

Because after the negative verb, the main verb appears in a special form called the connegative form.

Compare:

• uskon = I believe
• en usko = I do not believe

So the personal ending is carried by en, not by the main verb.

That is why you get:

• Minä uskon = I believe
• Minä en usko = I do not believe

not en uskon.

Finnish has no articles. There is no direct equivalent of English a/an or the.

Meaning is shown by context and by other words such as demonstratives like tämä = this.

So in this sentence:

• tätä = this in the required case
• uutista = news / news item in the required case

There is no need for a separate word meaning the.

Because tätä uutista is the object of the verb, and in a negative sentence Finnish normally puts the object in the partitive case.

So the basic dictionary forms are:

• tämä = this
• uutinen = news item, piece of news, report

But in this negative sentence they become:

• tätä
• uutista

This is very common in Finnish:

• Luen kirjan. = I read the book.
• En lue kirjaa. = I am not reading / I do not read the book.

Likewise:

• Uskon tämän uutisen. = I believe this news item.
• En usko tätä uutista. = I do not believe this news item.

They are both partitive singular forms.

1. tämä → tätä

This is an irregular-looking but very common change:

• tämä = this
• tätä = this (partitive)

2. uutinen → uutista

Words ending in -nen often change when they are inflected.

For example:

• uutinen = news item
• uutisen = of the news item
• uutista = news item (partitive)

This pattern is common with -nen words:

• nainen → naista
• ihminen → ihmistä
• uutinen → uutista

So tätä uutista means this news item in the partitive case.

Uutista is singular.

That can feel strange to an English speaker, because English news is grammatically singular but usually uncountable. Finnish works differently:

• uutinen = a news item, a piece of news, a report
• uutiset = the news, the news bulletin, news in general

So this sentence is talking about one specific report / piece of news:

• En usko tätä uutista. = I don’t believe this report / this piece of news.

Yes, Minä en usko tätä uutista is perfectly grammatical.

The version without minä is more neutral and common, because the pronoun is often omitted.

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

Examples:

• En usko tätä uutista. = neutral
• Minä en usko tätä uutista. = emphasizes I
• Tätä uutista en usko. = emphasizes this news
• Tätä uutista minä en usko. = strong contrast/emphasis

So the original sentence is a natural, neutral way to say it.

The dictionary form is uskoa, meaning to believe.

Some useful forms are:

• uskoa = to believe
• uskon = I believe
• uskoo = he/she believes
• usko = the verb form used after a negative verb
• en usko = I do not believe

So when you look it up in a dictionary, search for uskoa, not usko.

This is a very useful distinction.

uskoa jotakin

This means to believe something, in the sense of accepting a statement, report, or claim as true.

• Uskon tämän uutisen. = I believe this news item.
• En usko tätä uutista. = I don’t believe this news item.

uskoa johonkin

This usually means to believe in something or someone, often in the sense of faith, trust, or confidence.

Examples:

• Uskon Jumalaan. = I believe in God.
• Usko itseesi. = Believe in yourself.

So in your sentence, the direct object is used because the meaning is believe this report, not believe in this report.