Трудно судить по одному примеру.

No subject is missing. Трудно here is used impersonally and means it is hard or it’s difficult.

Russian often makes sentences like this without a word for it:

• Трудно понять. = It is hard to understand.
• Легко забыть. = It is easy to forget.

So Трудно судить по одному примеру literally works like:

• Hard to judge from one example

And in natural English:

• It’s hard to judge from one example.

After words like трудно, легко, можно, нельзя, Russian often uses the infinitive to say what is hard, easy, possible, or impossible to do.

So:

• трудно судить = hard to judge
• легко понять = easy to understand
• нельзя сказать = it’s impossible / you can’t say

The infinitive is the normal structure here.

Not necessarily. Судить can mean to judge, to form an opinion, or to conclude based on evidence.

In this sentence, it does not have to be about a courtroom. It means something like:

• to judge
• to assess
• to draw a conclusion

So the idea is: with only one example, it is hard to form a reliable judgment.

In this sentence, по means something like:

• from
• based on
• judging by
• on the basis of

So:

• судить по примеру = to judge from an example
• судить по словам = to judge by the words
• судить по внешности = to judge by appearance

This is a very common Russian pattern: судить по + dative.

Because по requires the dative case in this meaning.

The base form is:

• один пример = one example (nominative)

But after по, it changes to dative:

• по одному примеру = from one example / based on one example

Both words change because одному agrees with примеру:

• один → одному
• пример → примеру

Yes:

• по = preposition, here meaning from / based on
• одному = dative singular of один
• примеру = dative singular of пример

So the phrase literally means:

• according to one example
• from one example
• on the basis of one example

A learner should remember that по often takes the dative.

The imperfective судить is natural because the sentence describes a general action or process: judging in general.

Russian often uses the imperfective infinitive after words like трудно when speaking generally:

• Трудно понять
• Трудно объяснить
• Трудно судить

A perfective verb would usually sound more specific or less natural here. The sentence is about the general difficulty of making a judgment, not about completing one single judging action.

Yes. Russian word order is flexible, and the meaning stays basically the same.

Possible variants:

• Трудно судить по одному примеру.
• По одному примеру трудно судить.
• Судить по одному примеру трудно.

The first version is the most neutral and natural in many contexts.

Changing the order usually changes emphasis:

• По одному примеру трудно судить emphasizes from one example
• Судить по одному примеру трудно emphasizes the whole action judging from one example

Yes, often you can:

• Сложно судить по одному примеру.

This also means It’s hard to judge from one example.

The difference is small:

• трудно = difficult, hard
• сложно = difficult, complicated

In many everyday contexts they are close, but трудно sounds a bit more straightforward here.

The usual stress is:

• Тру́дно суди́ть по одному́ приме́ру.

A rough pronunciation guide:

• Тру́дно ≈ TROOD-na
• суди́ть ≈ soo-DEET’
• по одному́ ≈ pa a-da-NOO
• приме́ру ≈ pree-MYE-roo

One important thing for English speakers: unstressed о is often pronounced more like a in connected speech, so по одному may sound closer to pa adna-MOO than a careful spelling-based pronunciation would suggest.