Мы купили одну слойку в дорогу, а рулет оставили к выходным.

Because слойку is the direct object of купили (bought), so it has to be in the accusative case.

• Dictionary form: одна слойка
• In this sentence: одну слойку

Both words change to accusative feminine singular:

• одна → одну
• слойка → слойку

So Мы купили одну слойку literally means We bought one pastry.

Russian has no articles, so рулет can mean:

• a roll
• the roll

depending on context.

Also, Russian often leaves out один (one) unless the number itself matters. Here, the important contrast is:

• one pastry for the road
• the roll saved for the weekend

So рулет by itself is completely natural.

В дорогу is a very common expression meaning something like:

• for the road
• for the trip
• to take along on the journey

It does not mean that the pastry is physically moving into the road. It is an idiomatic phrase.

Examples:

• взять еду в дорогу = to take food for the trip
• купить что-то в дорогу = to buy something for the journey

Because the expression uses в with the accusative. In many cases, в + accusative suggests movement or direction, and in the fixed phrase в дорогу, the idea is roughly for setting off / for the trip ahead.

Here:

• дорога is the dictionary form
• в дорогу is accusative singular

So even though this is an idiom, the case choice still follows a normal Russian pattern.

In this sentence, а shows a contrast between the two parts:

• Мы купили одну слойку в дорогу
• а рулет оставили к выходным

It is not a strong contradiction like but in English can be. It is more like:

• and as for the roll...
• whereas the roll...
• but the roll...

Very roughly:

• и = and
• но = but with stronger opposition
• а = contrast, comparison, change of topic

So а is the most natural choice here.

Because оставили is perfective, and the sentence describes a completed action:

• they bought one item
• they left/saved the other item for later

That is a single finished event, so perfective fits well.

Compare:

• оставили = left / set aside / saved as a completed act
• оставляли = were leaving / used to leave / left repeatedly, depending on context

Here, the speaker is talking about what they did once, so оставили is the natural form.

К выходным means something like:

• for the weekend
• by the weekend
• until the weekend

The exact English wording depends on context, but the basic idea is that the roll was saved so that it would be there when the weekend came.

So оставили к выходным means they deliberately kept it for that later time.

It is the dative plural, because the preposition к normally takes the dative.

• dictionary form: выходные
• after к: к выходным

This is a standard pattern:

• к утру = by morning
• к вечеру = by evening
• к празднику = for the holiday / by the holiday
• к выходным = for the weekend / by the weekend

Because Russian usually treats the weekend as plural: literally, days off.

• выходной день = day off
• выходные = weekend / days off

So even when English uses singular weekend, Russian commonly uses plural выходные.

That is why you get forms like:

• на выходных
• к выходным
• до выходных

Russian word order is flexible, and here the object рулет is moved forward for contrast.

The sentence is structured like this:

• one pastry — for the road
• the roll — saved for the weekend

Putting рулет before the verb helps highlight that contrast after а.

A more neutral order like а оставили рулет к выходным is possible, but it sounds less pointed. The original order makes the comparison clearer.

Yes, it could be omitted if the subject is already clear from context. Russian often drops subject pronouns.

But in this sentence, Мы is helpful because the verbs are in the past plural:

• купили
• оставили

Those forms only tell you the subject is plural, not whether it means:

• we
• they
• sometimes even you plural/formal, depending on context

So Мы makes the subject explicit.

Yes, in many contexts you could, but the nuance is a little different.

• к выходным emphasizes by the time the weekend comes / until the weekend
• на выходные emphasizes for the weekend period

So:

• оставили к выходным = saved it until the weekend / for when the weekend comes
• оставили на выходные = left it for the weekend

Both can be natural, but к выходным has a slight feeling of holding it back until that later point.