W sierpniu jedziemy nad morze na wakacje.

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

The base form is sierpień = August.
After w, it changes to sierpniu.

So:

• sierpień = August
• w sierpniu = in August

This is a very common pattern:

• w maju = in May
• w lipcu = in July
• w sierpniu = in August

Polish often uses the present tense of an imperfective verb to talk about a planned or expected future event, especially when the time is already clear from the sentence.

Here, W sierpniu already tells you the action is future, so jedziemy naturally means we’re going / we’ll be going.

So this is normal Polish, not a mistake. It works much like English We’re going in August.

Polish distinguishes different kinds of going.

• jechać / jedziemy = to go by vehicle, to travel
• iść / idziemy = to go on foot

So jedziemy nad morze suggests travel by car, train, bus, etc., or simply treats it as a journey. If you were walking somewhere, you would normally use idziemy instead.

It means to the seaside, to the coast, or more literally to a place by the sea.

The preposition nad is used with places located by a body of water:

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

So it does not mean going into the sea. It means going to the area next to the sea.

This is a very important Polish pattern:

• movement toward a place → nad + accusative
• location in a place → nad + instrumental

So:

• Jedziemy nad morze. = We’re going to the seaside.
• Jesteśmy nad morzem. = We are at the seaside.

In other words:

• nad morze = destination
• nad morzem = location

It actually does change case, but the form happens to look the same.

Morze is a neuter singular noun, and for many neuter nouns the nominative and accusative forms are identical.

So here:

• nominative: morze
• accusative: morze

Same spelling, different grammatical role.

You can compare it with a feminine noun, where the change is visible:

• rzeka = river
• nad rzekę = to the river

Na wakacje means something like for vacation, on vacation, or for the holidays.

Polish often uses na + accusative when talking about going somewhere for a purpose, event, or period of time:

• na wakacje = for vacation
• na urlop = on holiday / on leave
• na weekend = for the weekend
• na koncert = to a concert

A useful comparison:

• jechać na wakacje = to go away for vacation
• w wakacje = during the summer holidays

So na wakacje focuses more on the trip as a vacation trip.

Because wakacje is a plural-only noun in Polish for summer vacation / summer holidays.

Polish treats this idea as plural, similar to how English often says the holidays.

So you normally say:

• wakacje są... = the vacation/holidays are...
• na wakacje = for vacation

There is no normal everyday singular wakacja with this meaning.

No. Polish has no articles like a, an, or the.

So Polish simply says:

• morze = sea / the sea
• wakacje = vacation / the vacation / holidays

Which meaning is intended is understood from context.

Yes. Polish word order is more flexible than English word order.

The sentence W sierpniu jedziemy nad morze na wakacje is very natural and starts by setting the time: In August...

Other orders are possible, for example:

• Jedziemy nad morze w sierpniu.
• Na wakacje jedziemy nad morze w sierpniu.

These are still understandable, but they shift the emphasis a little. The original version sounds neutral and natural.

In Polish, month names are not capitalized unless they begin a sentence.

So:

• styczeń
• maj
• sierpień

are all normally written with lowercase letters.

Also, morze is just a common noun, so it is lowercase too. A proper name would be capitalized, for example:

• Morze Bałtyckie
• Bałtyk