W przyszłym roku pojadę nad morze.

Because after w meaning in for time expressions, Polish uses the locative case.

• rok = year
• w roku = in the year
• w przyszłym roku = in the next year / next year

Both words change:

• przyszły → przyszłym
• rok → roku

So the whole phrase is in the locative.

It is the locative.

You can think of it like this:

• basic form: przyszły rok
• after w in a time expression: w przyszłym roku

This is a very common pattern:

• w tym tygodniu = this week
• w zeszłym miesiącu = last month
• w przyszłym roku = next year

Pojadę is the 1st person singular future form of pojechać, which means to go by vehicle / to set off and go somewhere by transport.

So pojadę means:

• I will go
• more literally: I will go (by car, train, bus, etc.)

In English, I’ll go to the seaside next year sounds natural, but in Polish the verb suggests travel by some means of transport, not going on foot.

Because pojechać is a perfective verb, and perfective verbs form the future with a single simple form.

So:

• jechać = imperfective, ongoing travel by vehicle
• pojechać = perfective, to go / set off / make the trip

Compare:

• Będę jechać nad morze = I will be traveling to the seaside / I will be on my way there
• Pojadę nad morze = I will go to the seaside

In your sentence, the idea is a planned future trip as a whole, so pojadę is the natural choice.

Polish often distinguishes how you go somewhere.

• iść = to go on foot, one specific trip
• chodzić = to go on foot habitually / repeatedly
• jechać = to go by vehicle
• jeździć = to go by vehicle habitually / repeatedly

So:

• Pójdę do sklepu = I’ll go to the shop (on foot)
• Pojadę do sklepu = I’ll go to the shop (by car, bus, etc.)

In W przyszłym roku pojadę nad morze, the speaker is talking about traveling somewhere, so pojadę fits very well.

Because nad morze means to the seaside / to the coast, not literally into the sea or to the sea as an object.

In Polish:

• nad morze = to the seaside, to the coast
• nad morzem = at the seaside, by the sea

Meanwhile:

• do morza literally means to the sea, but it is not the usual phrase for going on holiday to the coast.

So if you mean a trip to the coast, Polish normally says jechać nad morze.

Because after nad with motion toward a place, Polish uses the accusative case, and morze is a neuter noun whose accusative singular is the same as its nominative singular.

So:

• nominative: morze
• accusative: morze

That is why you see:

• jadę nad morze = I’m going to the seaside

But with location, not movement, nad takes the instrumental:

• jestem nad morzem = I am at the seaside

This is a very important contrast:

• nad morze = to the seaside
  → movement, direction
  → accusative
• nad morzem = at/by the seaside
  → location
  → instrumental

Examples:

• Latem pojadę nad morze. = In summer I’ll go to the seaside.
• Latem będę nad morzem. = In summer I’ll be at the seaside.

So in your sentence, nad morze is used because the person is going there.

Yes, you can, but usually ja is omitted unless you want emphasis.

Polish verbs already show the subject:

• pojadę = I will go

So:

• Pojadę nad morze. = neutral, normal
• Ja pojadę nad morze. = I will go to the seaside (emphatic, contrastive, or expressive)

For example:

• Ty zostaniesz w domu, a ja pojadę nad morze.
  = You’ll stay home, and I’ll go to the seaside.

Yes. Polish word order is flexible, although the original sentence is very natural.

Original:

• W przyszłym roku pojadę nad morze.

Other possible orders:

• Pojadę nad morze w przyszłym roku.
• Nad morze pojadę w przyszłym roku.

These are all grammatical, but the emphasis changes slightly.

A rough guide:

• W przyszłym roku pojadę nad morze = neutral, starting with the time
• Pojadę nad morze w przyszłym roku = neutral too, but the time comes later
• Nad morze pojadę w przyszłym roku = stronger focus on to the seaside

Not quite.

• w przyszłym roku is the normal way to say next year
• przyszłego roku can appear in some contexts, but it is not the usual basic equivalent of next year

For a learner, the safest and most natural choice is:

• w przyszłym roku

That is the standard everyday phrase.

A simple learner-friendly pronunciation guide is:

po-ja-dę

Approximate English-style hint:

• po like po in pole but shorter
• ja like ya
• dę has a nasal vowel at the end

More carefully:

• j in Polish sounds like English y
• ę is a nasal vowel; at the end of a word, many speakers pronounce it somewhat like e with nasalization, and in everyday speech it may sound close to em/en depending on context

So pojadę is roughly po-YA-de(n), but it is best to listen to native audio if possible.

You can use the same pattern with other expressions involving nad.

With movement toward a place:

• nad morze = to the seaside
• nad jezioro = to the lake
• nad rzekę = to the river

With location:

• nad morzem = at the seaside
• nad jeziorem = by the lake
• nad rzeką = by the river

So the pattern is very useful:

• motion → nad + accusative
• location → nad + instrumental

Because Polish often expresses the future through the verb form itself.

In English:

• I will go

In Polish:

• pojadę

The ending -ę tells you it is 1st person singular, and the perfective verb form makes it future.

So one Polish word can contain what English expresses with two words:

• pojadę = I will go

Yes, it is grammatical, but w przyszłym roku is usually the more natural and common way to say next year.

Compare:

• w przyszłym roku = the usual phrase for next year
• w następnym roku = more literally in the following year, sometimes sounding a bit more formal or context-dependent

For everyday speech, w przyszłym roku is the best choice here.