watasitati ha tiimusen ni sankasuru.

Normally, Japanese is written without spaces:

私たちはチーム戦に参加する。

The spaces here are just for learners, to make the parts easier to see. In real Japanese writing, you would usually not separate words this way.

は is the topic marker. It tells you what the sentence is about.

So in:

私たちはチーム戦に参加する。

the topic is 私たち — we / us.

A natural way to think about it is:

As for us, we will participate in the team competition.

It does not always mean simple emphasis on the subject. It mainly marks the topic being discussed.

Yes, it can often be omitted if the context already makes it clear who is participating.

Japanese frequently leaves out subjects and topics when they are understood. So depending on the situation, you might simply say:

チーム戦に参加する。

That could still mean We will participate in the team competition, if it is already obvious who we refers to.

私たち is included here to make the subject/topic explicit.

Usually, yes. 私たち literally means we or us.

• 私 = I / me
• たち = a suffix that makes it plural in this kind of use

So 私たち is we.

However, Japanese does not use pronouns in exactly the same way English does, so sometimes it can sound more explicit than necessary. In many situations, Japanese would just omit it.

チーム戦 means a team match, team competition, or team-based contest.

It is made of:

• チーム = team
• 戦 = battle, match, contest

The reading is チームせん.

This word is often used in contexts like:

• sports
• games
• tournaments
• competitions where people compete as teams rather than individually

So it suggests not just any event, but one where teams compete against each other.

Because 参加する normally takes に to mark the event, group, or activity that someone joins or participates in.

So:

• チーム戦に参加する = participate in the team competition
• 会議に参加する = participate in the meeting
• イベントに参加する = participate in the event

Here, に points to the thing being joined.

Because 参加する does not normally take を directly.

The usual pattern is:

X に 参加する
= participate in X

So:

• 大会に参加する
• 授業に参加する
• チーム戦に参加する

If you want to use を, you usually do it with the noun 参加 as part of another structure, such as:

• チーム戦への参加を決めた。
  We decided on participation in the team match.

But for the basic verb phrase, the natural form is:

チーム戦に参加する

Because Japanese is basically an SOV language: Subject/Topic + Object/Other information + Verb.

That means the main verb usually comes at the end of the sentence.

So in this sentence:

• 私たち = we
• は = topic marker
• チーム戦に = in the team competition
• 参加する = participate

This final-verb pattern is very normal in Japanese.

参加する is the plain form of the verb.
参加します is the polite form.

So:

• 私たちはチーム戦に参加する。 = plain style
• 私たちはチーム戦に参加します。 = polite style

The plain form is often used in:

• dictionary entries
• casual conversation
• written statements
• narration
• less formal contexts

The polite form is used when speaking politely to someone.

It can mean either, depending on context.

The plain non-past form する can express:

• a habitual action
• a general truth
• a future action

In this sentence, 参加する would often be understood as a future action:

We will participate in the team competition.

Japanese does not always mark future time separately the way English does. Context usually tells you.

参加する is a very common noun + する verb.

• 参加 = participation
• する = to do

Together, they mean to participate.

This pattern is extremely common in Japanese. For example:

• 勉強する = to study
• 運動する = to exercise
• 予約する = to make a reservation

So 参加する literally feels like to do participation, but in natural English it means to participate.

Yes, but the nuance changes.

• 私たちはチーム戦に参加する。
  This presents we as the topic: As for us, we will participate in the team competition.

• 私たちがチーム戦に参加する。
  This puts more focus on we as the ones who will participate, often with a contrast or emphasis such as we are the ones participating.

So both can be grammatically possible, but は is the more neutral choice if you are simply stating what we are going to do.

Because Japanese uses different writing systems for different kinds of words.

• チーム is a loanword from English team, so it is written in katakana
• 戦 and 参加 are standard Japanese words usually written in kanji

This kind of mix is completely normal in Japanese. A single sentence can naturally contain:

• kanji
• hiragana
• katakana

all together.