Фундук оказался свежим, и мы съели немного орехов ещё до ужина.

In Russian, фундук is very often used as a mass noun or as the name of the product/type of nut, like hazelnut or hazelnuts in a general sense.

So:

• Фундук оказался свежим = The hazelnuts turned out to be fresh / The hazelnut was fresh depending on context
• It does not necessarily mean there was only one nut

Russian often does this with foods and ingredients. The singular can refer to the substance or kind of food in general.

If you wanted to emphasize individual nuts, Russian might use something more explicitly countable, but фундук by itself is very natural here.

Оказался means turned out to be.

So Фундук оказался свежим means:

• The hazelnuts turned out to be fresh
• It turned out that the hazelnuts were fresh

This verb often expresses a result that became clear later, not just a simple state.

Compare:

• Фундук был свежим = The hazelnuts were fresh
  (just states a fact)
• Фундук оказался свежим = The hazelnuts turned out to be fresh
  (suggests this was discovered or confirmed)

Because Russian past tense agrees with the subject in gender and number.

Here the subject is фундук, which is:

• masculine
• singular

So the verb is:

• оказался = masculine singular past

Compare:

• фундук оказался
• вода оказалась
• орехи оказались

So if the subject were plural, you would get оказались.

After verbs like быть, стать, оказаться, казаться, and являться, Russian often uses the instrumental case for the predicate noun or adjective.

So:

• оказаться свежим = to turn out to be fresh

That is why you see:

• свежий → nominative
• свежим → instrumental singular masculine/neuter

This pattern is very common:

• Он оказался правым = He turned out to be right
• Она стала врачом = She became a doctor
• Это было странным = It was strange
  (less common than other patterns, but same idea)

So свежим is not random; it is required by the construction with оказаться.

Because съели is perfective, while ели is imperfective.

• есть / ели = to eat, were eating, used to eat
• съесть / съели = to eat up, eat completely, eat some and finish

In this sentence, the speaker refers to a completed action:

• мы съели немного орехов = we ate some nuts

The perfective form fits because the action happened and was completed before dinner.

If you said мы ели немного орехов, it would sound more like:

• we were eating some nuts
• we ate some nuts as an ongoing or background action

So съели is the more natural choice here.

Because немного normally takes the genitive case.

So:

• немного чего? — орехов

This is a very common Russian pattern:

• немного воды = a little water
• немного хлеба = a little bread
• немного яблок = a few apples
• немного орехов = some nuts / a few nuts

Since орехи is a plural count noun, the form after немного is genitive plural:

• nominative plural: орехи
• genitive plural: орехов

Yes, they refer to the same general thing, but from slightly different angles:

• фундук = hazelnut(s), the specific type
• орехи = nuts, the general category

Russian often does this naturally to avoid repeating the exact same word. So the sentence first identifies the food specifically:

• Фундук оказался свежим

Then it refers to what was eaten more generally:

• мы съели немного орехов

English does this too sometimes:

• The hazelnuts were fresh, and we ate some nuts before dinner.

If you wanted, Russian could also say мы съели немного фундука, but that sounds a bit more like we ate some hazelnut as a food substance. Немного орехов sounds very natural when thinking of actual nuts to snack on.

Here ещё adds the idea of even, already, or as early as.

So ещё до ужина means something like:

• even before dinner
• already before dinner
• as early as before dinner

It suggests that dinner had not happened yet, but they still ate some nuts beforehand.

This is a very common use of ещё: it adds emphasis that something happened earlier than expected or in addition to what might normally happen.

Because the preposition до requires the genitive case.

So:

• nominative: ужин
• genitive: ужина

That gives:

• до ужина = before dinner

This rule applies broadly:

• до дома = before reaching the house / up to the house
• до утра = until morning
• до конца = until the end

So ужина is simply the correct case after до.

Yes. Russian word order is fairly flexible, and the basic meaning would stay the same.

For example, you could also say:

• Фундук оказался свежим, и мы ещё до ужина съели немного орехов.
• Ещё до ужина мы съели немного орехов.

The original order:

• и мы съели немного орехов ещё до ужина

sounds natural and places ещё до ужина toward the end, where it adds a final time emphasis: even before dinner.

So the word order here is normal, but not the only possibility.