Learn Hardcore Russian: Однажды я поеду к морю с другом. - One day I will go to the sea with a friend.

the past:
- Однажды я встретил его в парке. – Once I met him in the park.
the future (as in your sentence):
- Однажды я поеду к морю с другом. – One day I will go to the sea with a friend.

In this sentence, the future meaning is clear because of the future verb поеду (I will go).

Why is it я поеду, not something like я буду ехать?

Both я поеду and я буду ехать can refer to the future, but they are not the same:

я поеду – perfective aspect (поехать)
- Focus on the fact of one trip starting / happening.
- Neutral, most common for a simple future plan:
  - Однажды я поеду к морю. – One day I will go (make a trip) to the sea.
я буду ехать – future of the imperfective (ехать)
- Focus on the process of going, what you will be doing at a certain time:
  - Завтра в три часа я буду ехать к морю. – Tomorrow at three I will be on my way to the sea.

In your sentence, we care about the fact of a future trip, not the process at a specific moment, so я поеду is the natural choice.

What is the difference between ехать, ездить, and поехать?

These all come from the same motion-verb family but have different meanings:

ехать – to go (by vehicle), unidirectional, imperfective
- One direction, in progress / general future:
- Я еду к морю. – I am going to the sea (now).
- Завтра я еду к морю. – Tomorrow I am going to the sea.
ездить – to go (by vehicle), multidirectional, imperfective
- Repeated trips, or movement in different directions:
- Я часто езжу к морю. – I often go to the sea.
поехать – to set off (by vehicle), perfective
- A single trip, focus on the start / fact of the trip:
- Я поеду к морю. – I will go (make a trip) to the sea.
- Я поехал к морю. – I went / set off to the sea.

In your sentence, поеду emphasizes one future trip.

Why is it к морю and not just море or something else?

Russian almost always needs a preposition to show direction. You cannot say я поеду море.

К is a preposition meaning towards / to (a place, a person):

к морю – to / toward the sea
к врачу – to the doctor
к дому – toward the house

So к морю is simply “to the sea”, with к showing movement in the direction of the sea.

Why is it морю and not море after к?

The preposition к always takes the dative case.

The noun море (sea) in the dative singular becomes морю:

Nominative (dictionary form): море – sea
Dative: морю – to the sea

So:

Я поеду к морю. – I will go to the sea.
Я подойду к окну. – I will approach the window. (окно → окну, also dative)

What is the difference between к морю, на море, and в море?

They all involve the sea, but with different prepositions and different ideas:

к морю – towards the sea / to the seaside
- Focus on movement toward the sea area:
- Я поеду к морю. – I will go to the sea (to the seaside).
на море – at the seaside / by the sea (location)
- Often about staying at a seaside resort:
- Я был на море. – I was at the seaside (on vacation by the sea).
в море – into the sea / in the sea (water)
- Physical position in the water itself:
- Корабль ушёл в море. – The ship went out to sea.
- Он плавал в море. – He swam in the sea.

Your sentence talks about going to the seaside, so к морю is natural.
You could also say Однажды я поеду на море with very similar meaning – more like One day I’ll go (on holiday) to the seaside.

Why is it с другом and not с друг?

The preposition с (with) normally takes the instrumental case when it means “together with someone”.

The noun друг (friend) in the instrumental singular becomes другом:

Nominative: друг – friend
Instrumental: другом – with (a) friend

Examples:

с другом – with a friend
с сестрой – with (my) sister
с учителем – with (the) teacher

So с другом literally means with (a) friend in the instrumental case.

What if the friend is female? Would с другом change?

If you specifically mean a female friend, you usually say подруга (female friend):

Nominative: подруга – female friend
Instrumental: подругой – with (a) female friend

So the sentence would be:

Однажды я поеду к морю с подругой. – One day I’ll go to the sea with a (female) friend.

С другом strongly suggests a male friend.
If you want to be more neutral in Russian, often context or another word (like товарищ, коллега) is used, but for “friend” itself Russian is gendered.

Which word is stressed in Однажды я поеду к морю с другом, and how do you pronounce it?

Stresses (marked with ´):

Одна́жды – od-NAZH-dy
я – ya
пое́ду – pa-YE-du
к мо́рю – k MO-ryu
с дру́гом – s DROO-gam

So the full sentence with stress:

Одна́жды я пое́ду к мо́рю с дру́гом.

Is the word order fixed? Can I move с другом or однажды?

Russian word order is more flexible than English. All of these are grammatical:

Однажды я поеду к морю с другом. (neutral)
Я однажды поеду к морю с другом.
Однажды с другом я поеду к морю. (slight emphasis on with a friend)
Я поеду к морю однажды с другом. (less natural, but still possible with context)

The basic neutral pattern is:

[Однажды] [я] [поеду] [к морю] [с другом].

Moving parts usually just changes emphasis or style, not the core meaning.

Why is there no word for “the” in к морю – “to the sea”?

Russian has no articles (no a/an, no the).

The noun море or морю can mean:

a sea
the sea
sea (in general)

Context tells the listener whether it is definite or indefinite.

So к морю can correspond to:

to the sea
to the seaside
to a sea (in a suitable context)

There is simply no separate word like the.

How would I say “Once I went to the sea with a friend” (past tense) using the same structure?

To talk about a single completed trip in the past, use the past tense of поехать:

For a male speaker:
- Однажды я пое́хал к мо́рю с дру́гом. – Once I went to the sea with a friend.
For a female speaker:
- Однажды я пое́хала к мо́рю с дру́гом.

You can also say с подругой if the friend is female, as explained before.