nakama to ryokou wo simasita.

Here, と means with.

So:

• 仲間 = companion(s), teammate(s), people in your group
• 仲間と = with my companions / with my group

In this sentence, と marks the person or people you did the action together with.

Compare:

• 仲間と旅行をしました。 = I traveled with my companions.
• 仲間は旅行をしました。 = As for my companions, they traveled.

So と is not the topic marker here; it shows accompaniment.

を marks the direct object of the verb.

Here, the object is 旅行:

• 旅行をしました

This works because 旅行 is a noun that can combine with する to make a verb-like expression:

• 旅行する = to travel / to take a trip
• 旅行をする = to do travel / to take a trip

So を shows that 旅行 is the thing being done.

Because 旅行 is a suru noun.

Many Japanese nouns can become verbs by adding する:

• 勉強する = to study
• 運動する = to exercise
• 旅行する = to travel

In polite past form:

• 旅行します = travel / will travel
• 旅行しました = traveled

You may also see:

• 旅行しました
• 旅行をしました

Both are correct.
The version with を makes the noun + する structure more explicit.

Yes, absolutely.

Both of these are natural:

• 仲間と旅行しました。
• 仲間と旅行をしました。

The version without を is often a little more straightforward and common in everyday use, but the version with を is also standard.

For a beginner, it is useful to recognize that both patterns exist with suru nouns.

Not exactly.

仲間 means something like:

• companion
• peer
• member of the same group
• teammate
• fellow

It often suggests people you belong with in some shared setting, such as:

• coworkers
• teammates
• club members
• friends in a group

So it is not always the same as 友達.

For example:

• 友達 = friend
• 仲間 = companion / one of your group

In English, the best translation depends on context.

Because Japanese often omits the subject when it is already understood from context.

So:

• 仲間と旅行をしました。

can mean:

• I traveled with my companions.
• We traveled with our group.
• He/She traveled with companions.

The exact subject depends on the situation.

This is very normal in Japanese. If the speaker wanted to be more explicit, they could say:

• 私は仲間と旅行をしました。 = I traveled with my companions.

But very often 私は is unnecessary.

しました is the polite past form of する.

The basic pattern is:

• する = do
• します = do / will do
• しました = did

So in this sentence:

• 旅行をしました = traveled / took a trip

Because it is in the past, the action already happened.

Yes.

The ending しました is polite, so the whole sentence is in polite style.

Compare:

• 仲間と旅行をしました。 = polite
• 仲間と旅行した。 = plain/casual

Learners should notice that Japanese verbs change depending on politeness level.

Yes, と can also mean and when it connects nouns.

For example:

• 犬と猫 = dogs and cats

But in this sentence, と means with because of the structure:

• 仲間と旅行をしました

It is attached to a person noun, and then followed by an action. That strongly suggests doing the action with someone.

So here:

• 仲間と = with companions

not:

• companions and travel

Japanese word order is more flexible than English, but particles show the role of each word.

This sentence is:

• 仲間と 旅行を しました。

Because of the particles:

• と marks the companion
• を marks the object

the roles are clear.

You may sometimes see slight rearrangements for emphasis, but the standard neutral order is exactly what you have here.

For beginners, it is best to learn it as:

person + と + noun + を + しました

Yes, you could, but the nuance is a little different.

• 仲間と旅行をしました。 = I traveled with my companions / I took a trip with my companions.
• 仲間と旅行に行きました。 = I went on a trip with my companions.

The second sentence focuses more on going on the trip, using 行きました.

The first sentence uses the suru noun 旅行 and simply states that the trip/travel happened.

Both are natural, but they are built differently.

It can mean either, depending on context.

Common translations are:

• travel
• trip
• journey

In a sentence like this, English usually prefers:

• I took a trip with my companions or
• I traveled with my companions

So 旅行 is broad enough to cover both the activity of traveling and the idea of a trip.

The spaces are usually just for learners.

Normal Japanese is written without spaces:

• 仲間と旅行をしました。

In beginner materials, spaces are often added to make it easier to see:

• where one word ends
• where a particle begins
• how the sentence is built

So the spaces are not part of standard Japanese writing.