Раз уж я везу щенка к ветеринару, я заодно куплю корм в зоомагазине.

Раз уж means something like since, seeing that, or if I’m already... It often has the nuance of:

• given that this is already happening
• since I’m doing this anyway
• might as well

So Раз уж я везу щенка к ветеринару... is not just a neutral since. It suggests: I’m already taking the puppy to the vet, so I may as well do one more useful thing.

Везти / везу means to transport by vehicle.

So:

• нести = to carry on foot, in your hands / on your body
• везти = to take/transport by car, bus, bicycle, etc.
• вести = to lead someone/something on foot

In this sentence, я везу щенка к ветеринару suggests the speaker is taking the puppy by some form of transport, most naturally by car.

Because щенка is the accusative singular form of щенок.

Щенок is:

• masculine
• animate
• singular

For animate masculine nouns, the accusative usually looks like the genitive:

• nominative: щенок
• accusative: щенка

Since the puppy is the direct object of везу (I’m taking the puppy), Russian uses щенка.

The preposition к means to / toward a person or place understood as a destination, especially when going to a person.

It requires the dative case, so:

• ветеринар → ветеринару

So:

• к ветеринару = to the vet

This is very natural in Russian when going to a doctor, specialist, friend, etc.:

• к врачу = to the doctor
• к другу = to a friend
• к ветеринару = to the vet

Because Russian often uses:

• present tense for something happening now or already underway
• perfective future for a single completed future action

So here:

• я везу щенка = I’m taking the puppy / I’m in the process of taking the puppy
• я куплю корм = I’ll buy food (one completed action)

The speaker is describing a current trip and then saying what they will do as part of it.

Заодно means something like:

• while I’m at it
• at the same time
• as well
• in the process

In this sentence, it connects the two errands:

• taking the puppy to the vet
• buying pet food

So я заодно куплю корм means I’ll buy food while I’m at it / while I’m out anyway.

Usually, yes. Корм most often means feed or pet food / animal food, not food for people.

So in this sentence:

• куплю корм в зоомагазине clearly means I’ll buy pet food at the pet store

For human food, Russian normally uses еда, пища, or the name of the specific food.

Because в зоомагазине here means in/at the pet store, indicating location.

With в:

• в + accusative = motion into somewhere
• в + prepositional = location inside somewhere

Compare:

• Я иду в зоомагазин. = I’m going to the pet store.
• Я куплю корм в зоомагазине. = I’ll buy food at the pet store.

In your sentence, the buying happens at that location, so Russian uses the prepositional case:

• зоомагазин → в зоомагазине

Russian often repeats the subject pronoun for clarity, balance, or emphasis, especially in longer sentences with two clauses.

So:

• Раз уж я везу щенка к ветеринару, я заодно куплю корм...

sounds natural and clear.

Could Russian omit the second я? Sometimes yes:

• Раз уж я везу щенка к ветеринару, заодно куплю корм...

That is also natural. But repeating я makes the structure a little more explicit and can sound slightly more deliberate.

Yes. The first clause sets up the reason/background:

• Раз уж я везу щенка к ветеринару...

Then the second clause gives the additional action:

• я заодно куплю корм в зоомагазине

This order feels very logical in Russian:

1. Since I’m already doing X
2. I’ll also do Y

Also, placing заодно before куплю highlights the idea of doing it at the same time / as an extra errand.

Yes, but the nuance would change a little.

• раз can mean since / if
• раз уж is more idiomatic here and adds the sense of since this is already the case anyway

Compare:

• Раз я везу щенка к ветеринару... = Since I’m taking the puppy to the vet...
• Раз уж я везу щенка к ветеринару... = Since I’m already taking the puppy to the vet anyway...

So раз уж sounds a bit more natural when the speaker is using one action as a reason to add another.

Куплю is the perfective future of купить.

Russian aspect matters because it tells you how the speaker sees the action:

• купить (perfective) = to buy, as a completed result
• покупать (imperfective) = to be buying / to buy habitually / to buy in general

So я куплю корм means:

• I’ll buy the food
• the speaker expects the action to be completed successfully

If you used an imperfective future like буду покупать, it would sound more like focusing on the process, which is not what this sentence wants.