Learn Hardcore Russian: Летом мы поехали в маленький порт на море. - In summer we went to a small port by the sea.

Летом comes from the noun лето (summer) and is in the instrumental case. In Russian, the instrumental is often used without a preposition to express time:

летом – in (the) summer
зимой – in (the) winter
весной – in (the) spring
осенью – in (the) autumn/fall

So Летом мы поехали… literally is like “By summer / in summer we went…”.

You cannot say ✘ в летом. To say “this summer”, you use an instrumental phrase like:

этим летом – this summer
прошлым летом – last summer

So the pattern is: [instrumental form of the time word] without a preposition.

Why do we use поехали and not ехали here?

Both come from the verb of motion ехать (to go by vehicle):

ехать – imperfective (process, background, “to be going”)
поехать – perfective (to set off, to go / go off once)

In the past tense:

мы поехали – we went / we set off (a single, completed trip; focus on the fact that the trip happened and reached its destination)
мы ехали – we were going / we were on our way (focus on the movement itself, or used as background to something else)

In a simple narrative sentence like Летом мы поехали в маленький порт…, Russian prefers the perfective поехали to tell a completed event in the story: That summer we went to a small port…

Мы ехали в маленький порт… would more naturally be:

“We were going to the small port…” (and then something else happened), or
Part of a description of the journey, not just a simple “we went”.

What is the difference between поехали and ездили in this context?

Ездить is the multidirectional / repeated-motion verb for “to go by vehicle”:

поехать – go (by vehicle) once, set off (perfective)
ездить – go (by vehicle) habitually, repeatedly, or back and forth (imperfective)

Compare:

Летом мы поехали в маленький порт на море.
→ That summer we went (once) to a small port on the sea.
Летом мы ездили в маленький порт на море.
→ In the summer we (used to) go / would go / went (repeatedly) to a small port on the sea.

So:

Use поехали for a single trip that happened and is viewed as one event.
Use ездили for repeated or habitual trips, or sometimes for a there-and-back trip as a whole (“we went there and back”).

Why is it в маленький порт and not в маленьком порту?

The choice comes from the question the verb answers:

куда? (to where?) → accusative case
где? (where?) → prepositional case

With поехали (we went), we are talking about destination → куда?:

в маленький порт – to a small port (accusative)

If you were describing a static location (where someone is), you would use the prepositional:

Мы были в маленьком порту. – We were in a small port.

So поехали в маленький порт is “went to a small port”, not “were in a small port”.

What case is маленький порт, and how do the adjective and noun agree?

The base noun is порт (port), which is:

Gender: masculine
Number: singular
Case here: accusative

For masculine inanimate nouns, the accusative form equals the nominative:

nominative: порт
accusative: порт

The adjective маленький (small) is also masculine singular accusative, and for adjectives, masculine accusative (inanimate) also looks like nominative:

nominative masculine sg: маленький порт
accusative masculine sg (inanimate): маленький порт

So they agree in:

gender: masculine
number: singular
case: accusative

Other examples in the same pattern:

большой порт – a big port
старый порт – an old port

If it were feminine, the accusative would change:
маленькая деревня (nom.) → в маленькую деревню (acc.).

What exactly does на море mean here? Why not к морю or в море?

In Russian, на море is a very common phrase, but it can mean slightly different things depending on context.

На море (with prepositional or accusative form море, which look the same) most often means:
- “by/at the sea, at the seaside”
  Especially in the context of vacations or coastal places:
- Мы поехали на море. – We went to the seaside.
- Отдыхать на море – to vacation at the sea / seaside.
К морю means “toward the sea” (direction towards, not necessarily arrival):
- Мы пошли к морю. – We walked towards the sea.
В море means “in the sea / out at sea” (inside the water or out on the open sea):
- Корабль вышел в море. – The ship went out to sea.
- Купаться в море. – To swim in the sea.

In в маленький порт на море, на море is most naturally understood as “on the sea / at the seaside”, describing where the port is (a seaport, not a river port). So the overall meaning is:

“to a small port on the sea (on the seacoast, by the sea)”

In this sentence, is на море part of the place we went to, or is it just describing the port?

It is mainly describing the port: a small port that is located on the sea.

You can imagine an expanded version:

Летом мы поехали в маленький порт, который находится на море.
– In the summer, we went to a small port that is on the sea.

So:

в маленький порт – destination (куда?)
на море – characteristic / location of that port (where that port is)

If you removed в маленький порт, then:

Летом мы поехали на море. – In the summer, we went to the seaside.

That would treat на море as the main destination instead of a description of the port.

Could I say у моря instead of на море? What is the difference between на море and у моря?

You can say both, but they are not identical.

на море – very idiomatic and broad: “at/by the sea, at the seaside”.
Often used for resorts or vacations:
Отдыхать на море. – To vacation at the seaside.
у моря – literally “by the sea / near the sea”.
This emphasizes physical proximity, standing or being next to the sea:
- Дом у моря. – A house by the sea.
- Гулять у моря. – To walk by the sea.

In your sentence:

в маленький порт на море – sounds very natural: a small port on the sea / on the seacoast.
в маленький порт у моря – would be understood as “a small port by the sea”, more literally “near the sea”, and is less idiomatic for “seaport”, though not impossible.

For a holiday destination, Russian speakers strongly prefer на море.

Can I change the word order to Мы летом поехали в маленький порт на море? Does that change the meaning?

Yes, you can say:

Мы летом поехали в маленький порт на море.

The basic factual meaning stays the same. The difference is in emphasis / what comes first as the topic:

Летом мы поехали…
– Fronts the time; “As for the summer / In the summer, we went…”
Мы летом поехали…
– Fronts мы; more like “We (and not someone else) went in the summer…”

Both orders are grammatically correct and natural; Russian word order is fairly flexible and used mostly to adjust focus rather than basic meaning.

How would I say “Every summer we went to a small port on the sea”? Can I still use поехали?

To express habitual/repeated action in the past, you normally use an imperfective verb, not поехали.

A natural sentence is:

Каждое лето мы ездили в маленький порт на море.
– Every summer we (used to) go / went to a small port on the sea.

Here:

каждое лето – every summer
ездили – went (repeatedly / habitually, by vehicle; imperfective)

Using поехали:

Каждое лето мы поехали… – sounds wrong/unnatural, because поехали is a one-time, completed event, but каждое лето clearly asks for a repeated/habitual action.

How are the words in this sentence stressed and pronounced?

Stresses:

ЛЕтом – stress on the first syllable: ЛЕ-там
мы – one syllable, unstressed in the sentence
поЕхали – stress on -Е-: па-ЙЕ-хали
в – single consonant [v]
мАленький – stress on мА-: МА-лень-кий
порт – one syllable, порт (strong [o])
на – one syllable [na]
мОре – stress on мО-: МО-ре

A rough phonetic approximation (in simple English-style transcription):

ЛЕтом мы поЕхали в мАленький порт на мОре.
→ “LYE-tam my pa-YE-ha-lee v MA-len-kee port na MO-re”

Knowing the stress is important because vowel quality changes a lot in unstressed syllables in Russian.