Следовательница долго смотрела на улику, но ничего не сказала.

Следовательница is a specifically feminine noun meaning female investigator.

• следователь = investigator, detective-like official; grammatically masculine by form
• следовательница = female investigator

Russian often has feminine forms for professions or roles, though usage depends on the word and on style. In modern Russian, some feminine profession words sound completely normal, while others may sound marked, informal, or stylistically loaded. Следовательница is understandable and clearly indicates the person is a woman.

That also explains why the verbs later are feminine: смотрела, сказала.

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

For singular subjects:

• masculine: смотрел, сказал
• feminine: смотрела, сказала
• neuter: смотрело, сказало
• plural: смотрели, сказали

Since следовательница is feminine, both past-tense verbs are feminine too:

• следовательница смотрела
• следовательница сказала

So the -ла ending tells you the subject is feminine singular.

Because смотреть на takes the accusative case.

The base form is:

• улика = clue, piece of evidence

Its accusative singular form is:

• улику

So:

• смотреть на улику = to look at the clue / evidence

This is just the pattern of the verb:

• смотреть на кого? что?
• look at whom? what?

More examples:

• смотреть на дом
• смотреть на девушку
• смотреть на фотографию

It can often be translated as either clue or piece of evidence, depending on context.

Usually улика leans more toward something incriminating or evidentiary in an investigation, so evidence is often the stronger translation. But in some learning contexts, clue is used because it sounds more natural in English.

So in this sentence, depending on the context, you could imagine:

• She looked at the piece of evidence for a long time...
• She looked at the clue for a long time...

Both can work, but piece of evidence is often closer.

Долго means for a long time. It refers to duration.

• долго смотрела = looked for a long time

English speakers sometimes expect something related to long, but in Russian the normal adverb for duration is долго.

Compare:

• долгий = long, lengthy
• долго = for a long time

Длинный / длинно usually refers more to physical length, not duration:

• длинная улица = a long street

So долго смотрела is the natural choice here.

This is a question of aspect.

• смотреть = imperfective
• посмотреть = perfective

In this sentence, смотрела is used because the action is presented as an ongoing process or duration:

• долго смотрела = was looking / looked for a long time

That fits the imperfective very well.

If you said посмотрела, it would sound more like took a look or looked once / briefly / as a completed act, which clashes with долго unless the context is special.

So:

• долго смотрела = natural
• долго посмотрела = much less natural in a basic sentence like this

Because сказать and говорить are not used in exactly the same way.

Here:

• сказать = to say, to utter, to state
• говорить = to speak, to be speaking, to talk

In но ничего не сказала, the idea is that she did not make any verbal statement at all. That is a completed, bounded event, so the perfective сказать is natural.

• ничего не сказала = said nothing / didn’t say anything

If you used не говорила, it would sound more like was not speaking or did not speak, which is possible in some contexts but not the most direct match here.

So the contrast is very natural:

• she was looking for a long time
• but said nothing

Russian normally uses double negative.

So where English says:

• She didn’t say anything or
• She said nothing

Russian usually says:

• Она ничего не сказала

Literally, it looks like nothing not said, but that is just standard Russian grammar.

Important point: if you use a negative pronoun like ничего, the verb is also negative:

• никто не пришёл = nobody came
• ничего не понимаю = I understand nothing / I don’t understand anything
• никогда не говорит = he never speaks

This is one of the most important differences from English.

Ничего is the form used here for nothing.

Historically and grammatically, it comes from ничто and is used in oblique-case forms. After negation, Russian often uses forms that match the genitive pattern, and ничего is the standard object form in sentences like this:

• ничего не сказала
• ничего не видел
• ничего не знаю

For a learner, the most practical thing is to memorize ничего не + verb as a very common pattern meaning didn’t ... anything / ... nothing.

Because in Russian, just like in English, you often do not repeat the subject if it stays the same.

The sentence is:

• Следовательница долго смотрела на улику, но ничего не сказала.

The second clause still refers to the same person, so repeating следовательница would be unnecessary.

You could technically repeat it for emphasis or style:

• Следовательница долго смотрела на улику, но следовательница ничего не сказала.

But that sounds unnatural and overly repetitive in normal usage.

Но means but and links two contrasting clauses.

Here the contrast is:

• she looked at the evidence for a long time
• but she said nothing

So:

• ..., но ничего не сказала.

Its placement is very similar to English but: it usually comes between the two clauses it connects.

Yes. Russian word order is flexible, though the neutral version here is very natural:

• Следовательница долго смотрела на улику, но ничего не сказала.

You could change the order for emphasis. For example:

• Долго следовательница смотрела на улику, но ничего не сказала.
  Emphasizes for a long time

• На улику следовательница долго смотрела, но ничего не сказала.
  Emphasizes the evidence/clue

But the original sentence is the best neutral order for most contexts.

Yes: the comma before но is standard.

Russian, like English, normally uses a comma before coordinating conjunctions such as но when they join two clauses with their own verbs:

• смотрела
• сказала

So the comma is required here:

• Следовательница долго смотрела на улику, но ничего не сказала.