Мне не верится, что он ушёл.

Because this sentence uses an impersonal construction.

Russian often expresses feelings or mental states as something that happens to a person, rather than something the person actively does. So мне is in the dative case and marks the experiencer:

• мне = to me / for me

So Мне не верится is literally something like:

• To me, it doesn’t seem believable
• or more naturally, I can’t believe it

This is different from a straightforward personal sentence like Я не верю....

Верится comes from the verb вериться, which is related to верить (to believe), but it is used in an impersonal way.

A useful way to think of it:

• верить = to believe
• вериться = to feel believable / to be easy or possible to believe

So:

• Мне верится = It feels believable to me
• Мне не верится = It doesn’t feel believable to me / I can’t believe it

In real usage, верится / не верится is very common in this impersonal pattern with the dative.

They are close, but not identical.

• Я не верю = I do not believe

  • more direct
  • can mean you think something is false
  • can sound more deliberate or skeptical

• Мне не верится = I can’t believe it

  • more emotional
  • often used when something is surprising, shocking, or hard to accept as real
  • does not always mean you think it is false; it may mean you know it is true, but it still feels unbelievable

So in this sentence, Мне не верится, что он ушёл usually suggests:

• I know he left, but it still feels unreal to me

The -ся ending is historically reflexive, but here it is not literally reflexive in the English sense.

In Russian, -ся often does more than mean oneself. It can help form:

• passive-like meanings
• middle meanings
• impersonal expressions

Here it helps create an impersonal state:

• верить = to believe
• вериться = to seem believable / to come naturally to believe

So you should not translate верится word-for-word as believes itself. It is better to learn мне не верится as a set expression meaning I can’t believe it.

Because impersonal verbs in Russian are often put in the 3rd person singular neuter-like form, even when there is no real subject.

That is very common in Russian:

• Мне хочется = I feel like...
• Мне не спится = I can’t sleep
• Мне не верится = I can’t believe it

There is no normal subject like я controlling the verb here. The person experiencing the feeling is in the dative (мне), and the verb stays in this impersonal singular form.

Что introduces a content clause — the thing that is being believed, said, known, etc.

So:

• Мне не верится, что он ушёл
• literally: It’s hard for me to believe that he left

The clause что он ушёл explains what is hard to believe.

This is very similar to English that in sentences like:

• I know that...
• I think that...
• I can’t believe that...

Because ушёл is perfective, and here Russian is talking about a completed event: he left, and now he is gone.

• уходить = imperfective
• уйти = perfective
• ушёл = past tense of уйти

Using ушёл focuses on the result:

• he left
• the action is complete

If you used уходил, it would usually suggest something different, such as:

• the process of leaving
• repeated leaving
• temporary going away in some contexts
• background description rather than a completed result

In this sentence, the natural idea is the completed fact that he has left / he is gone, so ушёл is the right choice.

Because the subject is он, which is masculine singular.

In the past tense, Russian verbs agree with gender and number:

• он ушёл = he left
• она ушла = she left
• оно ушло = it left
• они ушли = they left

So ушёл matches он.

Yes. Russian word order is flexible, and changing it usually changes emphasis, not the basic meaning.

Neutral order:

• Мне не верится, что он ушёл.

Possible alternatives:

• Что он ушёл, мне не верится.
  • stronger emphasis on the fact that he left
• Он ушёл? Мне не верится.
  • more conversational, split into two parts

The original sentence is a very natural neutral way to say it.

Yes, very common.

You can say it by itself when reacting to surprising news:

• Мне не верится! = I can’t believe it!

You can also expand it with a clause:

• Мне не верится, что это правда.
• Мне не верится, что он уже здесь.
• Мне не верится, что он ушёл.

So this sentence follows a very common Russian pattern.

Yes, absolutely. That is another very common way to express the same general idea.

• Мне не верится, что он ушёл = more emotional, impersonal, “it feels unbelievable”
• Не могу поверить, что он ушёл = more direct, “I can’t believe that he left”

Both are natural. The version with мне не верится often feels slightly softer or more expressive of inner reaction rather than a simple statement of inability.

Not necessarily.

That is an important nuance.

Мне не верится often means:

• I know it’s true
• but it is shocking, unexpected, or emotionally hard to accept

So the speaker is often not denying the fact. They are saying the fact feels unreal.

That is why this expression is especially common for strong reactions to news, change, loss, surprise, or disbelief in the emotional sense.