На пикник мы купили по две сосиски и немного кетчупа.

Here на + accusative means for a picnic / for the picnic in the sense of purpose or intended use.

In Russian, на is often used this way with things you buy or prepare for an event:

• купить еду на ужин = buy food for dinner
• взять что-то на работу = take something to work
• приготовить салат на праздник = make a salad for a celebration

So На пикник мы купили... means For the picnic, we bought...

It does not necessarily mean physical movement onto or to something here. It is more like for the occasion.

Yes, мы could be omitted.

Russian often leaves out subject pronouns when they are clear from the verb:

• купили already means we bought

So both are possible:

• На пикник мы купили...
• На пикник купили...

Including мы gives a bit more emphasis or contrast, as if saying as for us, we bought... It can help make the sentence clearer or more natural in context.

Here по is a distributive word. It means something like:

• two sausages each
• two sausages per person

So мы купили по две сосиски means that each person got two sausages, not just two sausages in total.

This use of по is very common:

• по одному билету = one ticket each
• по три яблока = three apples each
• по две чашки = two cups each

So this is not the preposition по in the sense of along, according to, etc. Here it means apiece / each.

Because сосиска is a feminine noun.

The number 2 has two forms in Russian:

• два for masculine and neuter nouns
• две for feminine nouns

Examples:

• два стола = two tables
• два окна = two windows
• две книги = two books
• две сосиски = two sausages

Since сосиска is feminine, the correct form is две.

After the numbers 2, 3, 4, Russian normally uses the noun in the form that looks like the genitive singular:

• две сосиски
• три сосиски
• четыре сосиски

Compare:

• одна сосиска
• две сосиски
• пять сосисок

So after две, сосиска becomes сосиски.

This is one of the standard number patterns in Russian:

• 1 → nominative singular
• 2–4 → genitive singular form
• 5+ → genitive plural

It helps to think of it as a combination of two patterns:

1. две сосиски = the normal numeral phrase two sausages
2. по adds the idea of each / apiece

So:

• две сосиски = two sausages
• по две сосиски = two sausages each

The important thing is that по changes the meaning of the whole phrase, making it distributive.

Because немного is followed by the genitive.

So:

• немного воды = a little water
• немного хлеба = a little bread
• немного кетчупа = a little ketchup

Кетчуп is a masculine noun, and its genitive singular form is кетчупа.

This is very common with words expressing an indefinite amount:

• много
• мало
• немного
• достаточно

All of these usually require the noun in the genitive.

Because кетчуп is being treated as an uncountable mass noun, like water or bread in English.

You are not counting separate ketchups. You are just talking about some amount of ketchup.

So Russian uses singular:

• немного кетчупа = a little ketchup

If you were talking about countable units, the structure would be different, for example:

• несколько бутылок кетчупа = several bottles of ketchup

Russian word order is flexible, so the beginning of the sentence often shows the topic or the thing the speaker wants to frame first.

Starting with На пикник sets the context:

• As for the picnic...
• For the picnic...

So the sentence feels like:

For the picnic, we bought two sausages each and a little ketchup.

A different order is also possible, for example:

• Мы купили на пикник по две сосиски и немного кетчупа.

That version is also natural. The original just puts more focus on the picnic as the situation or purpose.

Sometimes literally на can look like to, but here the most natural interpretation is for the picnic.

If you translate it as to the picnic, it may sound in English as if the important idea is movement or destination. In this sentence, the idea is really what we bought for that occasion.

So for a learner, for the picnic is the safest and most helpful understanding here.

Not exactly.

Сосиска usually means a small, soft sausage, often the kind that can be boiled or heated quickly. In many contexts it can correspond to:

• sausage
• hot dog sausage
• frankfurter-type sausage

So сосиски is often broader than just one exact English word. The precise English translation depends on context.

In this sentence, sausages is a perfectly good practical translation.

Yes, in informal writing you may see:

• по 2 сосиски

But in normal careful writing, spelling out the number is often preferred:

• по две сосиски

The grammar is the same either way. The choice is mainly about writing style.

It sounds natural.

A Russian speaker could easily say:

• На пикник мы купили по две сосиски и немного кетчупа.

It is a normal everyday sentence with very common structures:

• на + event/purpose
• по + numeral for each
• немного + genitive for a little of something

So it is a good sentence to learn from because it contains several useful real-life patterns.