Перед работой я быстро съел мандарин, а банан оставил на вечер.

Because перед normally requires the instrumental case when it means before / in front of.

So:

• работа = nominative
• работу = accusative
• работой = instrumental

That is why you get перед работой = before work.

This is a very common pattern:

• перед уроком = before class
• перед встречей = before the meeting
• перед сном = before sleep / before bed

In Russian, case after a preposition is determined by the preposition’s grammar, not just by the English meaning.

Even though перед работой is translated as a time expression (before work), Russian still uses the same preposition перед, and перед takes the instrumental case.

So Russian treats this as a normal перед + instrumental construction.

They are both past tense verbs:

• съел = ate up / finished eating
• оставил = left / set aside

Russian past tense is formed from the infinitive stem plus past endings. In the singular, past tense also shows gender:

• masculine: съел, оставил
• feminine: съела, оставила
• neuter: съело, оставило

So this sentence, as written, sounds like it was said by a male speaker.
If the speaker were female, it would be:

Перед работой я быстро съела мандарин, а банан оставила на вечер.

Because съесть is perfective, while есть is imperfective.

• ел = was eating / ate (process, no emphasis on completion)
• съел = ate up / finished eating (completed action)

In this sentence, the idea is that the speaker ate the tangerine as a completed action before work, so съел fits well.

Compare:

• Я ел мандарин. = I was eating / I ate a tangerine.
• Я съел мандарин. = I ate the tangerine up / I finished a tangerine.

Because мандарин is a masculine inanimate noun, and here it is the direct object.

For masculine inanimate nouns:

• nominative = мандарин
• accusative = мандарин

So the accusative looks the same as the nominative.

That is why:

• Я съел мандарин. = I ate a tangerine.

But with an animate masculine noun, accusative would often match the genitive instead.

Russian has no articles like a or the.

So мандарин can mean:

• a tangerine
• the tangerine

The exact meaning depends on context.

In this sentence, English would often naturally say a tangerine and the banana or a banana, depending on context. Russian leaves that unspecified unless the speaker adds something else.

If Russian wants to be more specific, it can use words like:

• этот мандарин = this tangerine
• один мандарин = one / a single tangerine

The conjunction а often shows a contrast or comparison, especially when two things are treated differently.

Here the contrast is:

• the tangerine was eaten quickly
• the banana was saved for the evening

So а is very natural.

Roughly:

• и = and, simple addition
• но = but, stronger contradiction
• а = and / while / whereas, with contrast

In this sentence, а is the best choice because it sets up a side-by-side contrast.

Because на вечер means for the evening, while вечером means in the evening.

Those are different ideas:

• оставил на вечер = left it for the evening, saved it until evening
• оставил вечером = left it in the evening

So here на вечер expresses purpose or intended time.

Similar examples:

• оставить что-то на завтра = leave something for tomorrow
• перенести встречу на понедельник = move the meeting to Monday

It is in the accusative case: на вечер.

With на, Russian can use different cases depending on meaning. Here it expresses movement toward a time slot or allocation for a time, so accusative is used.

Compare:

• на вечер = for the evening
• на завтра = for tomorrow
• на утро = for the morning

Yes, Russian word order is flexible.

Я быстро съел мандарин is a neutral, natural way to say I quickly ate a tangerine.

You could also say:

• Я съел мандарин быстро
• Быстро я съел мандарин

But these may sound different in emphasis or style. The version in the sentence is the most neutral and natural for everyday speech.

In Russian, word order often helps show focus, contrast, or what is already known.

Yes, often it can be omitted if the subject is clear from context.

Russian frequently drops subject pronouns:

• Перед работой быстро съел мандарин, а банан оставил на вечер.

This can sound perfectly natural in conversation if it is obvious who did it.

However, keeping я is also completely normal. It can make the sentence clearer or slightly more explicit.

Because the sentence is talking about one tangerine and one banana.

Russian singular and plural work much like English here:

• мандарин = tangerine
• мандарины = tangerines
• банан = banana
• бананы = bananas

So the sentence specifically presents two individual items.

Both are possible, but they feel a little different.

• мандарин = neutral
• мандаринка = diminutive, often warmer, cuter, more conversational

So:

• Я съел мандарин = neutral
• Я съел мандаринку = I ate a little tangerine / a tangerine, with a more affectionate or informal tone

The original sentence is neutral and straightforward.

Because а is joining two clauses:

• Перед работой я быстро съел мандарин
• а банан оставил на вечер

In Russian, conjunctions like а, но, and often и between clauses are usually separated by a comma.

So the comma is standard punctuation here.

Yes, that is a good more literal reading.

A few shades of meaning are worth noticing:

• съел suggests completion: ate up / finished eating
• а suggests contrast: whereas / while / and as for
• оставил на вечер means saved for the evening, not just physically left somewhere

So a very close English rendering might be:

Before work I quickly ate a tangerine, but saved the banana for the evening.

That captures the contrast nicely.