Наш сосед возит овощи на рынок каждую субботу, а сегодня он везёт только картофель.

This is one of the classic Russian verbs of motion patterns.

• возить = to transport/carry something regularly, repeatedly, or in general
• везти = to transport/carry something right now, in one particular trip, in one direction

So:

• Наш сосед возит овощи на рынок каждую субботу = this is his usual habit
• сегодня он везёт только картофель = today, on this specific trip, he is taking only potatoes

English often just uses take or bring in both cases, but Russian makes this distinction very clearly.

Yes. Both are imperfective, but they belong to different motion patterns:

• везти = determinate / one-directional transport
• возить = indeterminate / multidirectional / habitual transport

That means Russian is not contrasting completed vs incomplete here. Instead, it is contrasting:

• habit / repeated action → возит
• one current trip → везёт

So the difference is not mainly aspect in the usual perfective/imperfective sense, but the special behavior of verbs of motion.

Usually, yes, or at least it suggests transporting something rather than simply carrying it in your hands on foot.

Compare:

• нести / носить = carry on foot
• везти / возить = transport, often by car, cart, truck, etc.

So сосед возит овощи на рынок sounds like the neighbor brings them there as transported goods, probably by some kind of vehicle.

Because Russian uses на with some destinations, and рынок is one of them.

So:

• на рынок = to the market
• на рынке = at the market

This is similar to other places that use на, such as:

• на почту = to the post office
• на вокзал = to the station
• на работу = to work

You usually just have to learn which nouns take в and which take на.

They are the direct objects of the verbs, so they are in the accusative case.

• овощи is plural
• картофель is singular

Because these nouns are inanimate, their accusative forms look like the nominative forms:

• овощи → accusative plural овощи
• картофель → accusative singular картофель

If the object were animate, the accusative might look different.

Because Russian often uses the accusative case for expressions of time meaning every X, this X, etc.

So:

• каждую субботу = every Saturday

Here суббота is a feminine noun, and its accusative singular is субботу.

Compare:

• каждую неделю = every week
• каждое утро = every morning
• каждый день = every day

So каждую субботу is not the subject. It is a time expression in the accusative.

а often marks a contrast or a switch in topic, but not always a strong contradiction.

Here it means something like:

• ..., whereas today...
• ..., but today...

So the sentence contrasts:

• the usual situation: he takes vegetables every Saturday
• today’s special situation: today he is taking only potatoes

Why not the others?

• и would simply add information: and
• но would sound more like a stronger but / however
• а is very natural for this kind of side-by-side contrast

Yes, Russian can sometimes omit subject pronouns when the meaning is obvious from the verb form and context. So a shorter version could be:

Наш сосед возит овощи на рынок каждую субботу, а сегодня везёт только картофель.

That is possible.

But он is very natural here because it helps with:

• clarity
• rhythm
• contrast

It makes the second clause feel neatly parallel to the first one.

Both are possible, but they are a little different in style.

• картофель = more neutral, formal, or standard
• картошка = more everyday, conversational

So:

• везёт только картофель sounds slightly more neutral or business-like
• везёт только картошку sounds more colloquial

A learner should recognize both.

Because овощи means vegetables as a category, while картофель can function like a mass noun, similar to potato produce rather than individual potatoes.

Russian often uses:

• овощи = vegetables
• картофель = potatoes, potato crop, potato as a product

In everyday speech, people also very often say картошка.

So this is normal vocabulary usage, not a grammar mistake.

They are both present tense, third person singular:

• возит = he transports regularly / he takes
• везёт = he is taking / he is bringing / he is transporting

In Russian, present tense can cover both:

• simple present: he takes
• present progressive: he is taking

The surrounding words show which meaning is intended:

• каждую субботу makes it habitual
• сегодня makes it a current specific situation

Yes. Russian word order is flexible, and changing it changes the focus or emphasis more than the core meaning.

For example:

• Сегодня он везёт только картофель.
  Neutral: Today he is taking only potatoes.

• Только картофель он сегодня везёт.
  Stronger emphasis on only potatoes.

• Он сегодня везёт только картофель.
  Slight emphasis on today as part of the statement about him.

The original sentence is very natural and balanced, especially because it contrasts a habitual action with today’s exception.

It literally means our neighbor.

• наш = our
• сосед = male neighbor

In Russian, possessives like наш, мой, его, etc. are used very naturally in places where English might or might not use them.

So наш сосед is a normal way to introduce someone known to the speakers.

If the neighbor were female, it would be:

• наша соседка

Only the words that agree with gender would change:

• Наш сосед → Наша соседка
• он → она

The verbs would stay the same in the present tense:

• Наша соседка возит овощи на рынок каждую субботу, а сегодня она везёт только картофель.

In the present tense, Russian verbs do not change for gender. Gender matters in the past tense, but not here.

Yes. Like many motion verbs, the best English translation depends on perspective.

везти can be:

• to take
• to bring
• to transport

In this sentence, English might say either:

• Today he is taking only potatoes
• Today he is bringing only potatoes

Russian itself does not force exactly the same viewpoint distinction that English often does with take vs bring. The important thing is the motion/transport meaning.