Мы специально взяли с собой термос, потому что вечером у воды прохладно.

It means took with us / brought along.

Russian often uses взять с собой as a set expression:

• взять = to take
• с собой = with oneself / along

So Мы взяли с собой термос is very naturally We took a thermos with us or We brought a thermos along.

Because взяли is the perfective past form of взять, and it presents the action as a completed whole: we took it.

Compare:

• брали (imperfective) = were taking / used to take / took in an ongoing or repeated sense
• взяли (perfective) = took, successfully and as a completed action

In this sentence, the speaker is talking about one completed action: they made sure to take a thermos along. That is why взяли fits better.

Because the subject is мы = we.

In the past tense, Russian verbs agree with the subject in gender and number:

• взял = he took
• взяла = she took
• взяло = it took
• взяли = they took / we took

Russian uses the same plural past form -ли for both we and they.

Because с собой refers back to the subject and means with oneself / with ourselves / with themselves, depending on the subject.

Here:

• Мы взяли с собой термос = We took a thermos with us

If you said с нами, that would literally mean with us, but after взять Russian normally prefers the reflexive expression взять с собой.

So:

• взять с собой = the natural idiom for take along
• собой is the instrumental form of себя

Here специально means deliberately, on purpose, or specifically for that reason.

So the idea is:

• Мы специально взяли с собой термос = We deliberately brought a thermos along / We made a point of taking a thermos with us

It suggests they did this intentionally because they expected it to be cool later.

Термос is in the accusative case because it is the direct object of взяли.

However, for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: термос
• accusative: термос

That is why there is no visible change.

Because потому что means because and introduces a subordinate clause.

Russian normally puts a comma before subordinate clauses:

• Мы специально взяли с собой термос, потому что вечером у воды прохладно.

This is very standard punctuation.

Because вечером is the instrumental form used adverbially to mean in the evening.

This is a very common Russian pattern for time expressions:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So вечером does not literally mean by evening here; it simply means in the evening.

Literally, у воды means by the water or near the water.

It usually refers to being near a body of water, such as:

• a river
• a lake
• the sea
• the shore

So in natural English, this part often becomes:

• by the water
• near the water
• by the lakeside / riverside / shore, depending on context

Because the preposition у normally takes the genitive case.

The noun вода changes like this:

• nominative: вода
• genitive: воды

So:

• у воды = by the water / near the water

This is a very common pattern:

• у дома = by the house
• у окна = by the window
• у реки = by the river

Because прохладно here is not an adjective describing a noun. It is a predicative adverb/state word meaning it is cool.

Russian often uses this kind of impersonal construction for weather and environmental conditions:

• холодно = it is cold
• жарко = it is hot
• темно = it is dark
• прохладно = it is cool

So:

• вечером у воды прохладно = it’s cool by the water in the evening

If you used прохладный / прохладная, you would need a noun:

• прохладный вечер = a cool evening
• прохладная вода = cool water

No, Russian word order is fairly flexible, but the given order is natural and neutral.

Current order:

• Мы специально взяли с собой термос, потому что вечером у воды прохладно.

This sounds like:

1. who did it
2. what they intentionally did
3. why

You could move things around for emphasis, for example:

• Мы взяли с собой термос специально, потому что вечером у воды прохладно.
• Потому что вечером у воды прохладно, мы специально взяли с собой термос.

These are possible, but the original sentence is the most straightforward and idiomatic in ordinary speech.

No.

• у воды = by the water / near the water
• в воде = in the water

So this sentence means it gets cool near the water, not in the water.

That distinction is important:

• сидеть у воды = sit by the water
• стоять в воде = stand in the water

Yes, basically. Термос means a thermos flask, vacuum flask, or insulated container for hot or cold drinks.

In this sentence, it strongly suggests they brought it so they could have something warm later, since it gets cool in the evening.