watasi ha koukou, tomodati ha senkou da.

A natural reading is:

わたし は こうこう、ともだち は せんこう だ。

Word by word:

• 私 = わたし
• は = topic marker, pronounced わ
• 後攻 = こうこう
• 友達 = ともだち
• 先攻 = せんこう
• だ = plain-form copula

These words are used mainly in games, matches, and turn-based situations.

• 先攻 = going first / the first player / the side that moves first
• 後攻 = going second / the second player / the side that moves second

So this sentence is saying that I am the second player, and my friend is the first player.

These are not just general words for first and second in every situation. They are specifically about order of play.

Because the sentence is comparing two topics:

• 私は後攻
• 友達は先攻だ

This structure sets up a contrast:

• As for me, I go second.
• As for my friend, they go first.

Using は twice is very common when contrasting two people or things. In English, we often show this with stress:

• I go second, and my friend goes first.

Because in Japanese, the copula in the first of two linked clauses is often omitted.

So:

• 私は後攻、友達は先攻だ。

is essentially a shortened version of something like:

• 私は後攻だ、友達は先攻だ。

But repeating だ in the middle can sound stiff or unnatural. Japanese often leaves it out before continuing to the next clause.

This omission is very normal in casual and written Japanese.

The final だ closes the whole statement in plain style.

Japanese often saves the sentence-ending element for the end, especially when multiple noun-predicate clauses are lined up. So the pattern is:

• A は B、C は D だ。

This is a very common way to state two parallel facts.

Yes, absolutely.

• 私は後攻、友達は先攻だ。
• 私は後攻で、友達は先攻だ。

Both are natural.

The version with で makes the connection between the two parts more explicit, like and in English. The version without で is a bit more compact and crisp.

Sometimes, but it changes the nuance.

• 私は後攻、友達は先攻だ。
  This sounds like a contrast: I am second, while my friend is first.

If you said:

• 私が後攻で、友達が先攻だ。

it would feel more like identifying who fills each role, as if answering Who is going second? Who is going first?

So:

• は = topic/contrast
• が = identification/emphasis on the subject

In this sentence, は is the more natural choice.

Yes, it could be omitted if the context is clear.

For example:

• 後攻、友達は先攻だ。

or even:

• 私は後攻、先攻は友達だ。

But Japanese often omits obvious subjects and topics. If everyone already knows you are talking about yourself, 私 may not be needed.

Still, including 私 makes the contrast very clear, so the original sentence is perfectly natural.

The comma marks a pause between the two contrasted parts:

• 私は後攻、
• 友達は先攻だ。

It helps readability and makes the structure clearer. Japanese punctuation is often more flexible than English punctuation, so you may see slight variations, but the comma is very normal here.

In this sentence, 友達 most naturally means friend in the singular because it is paired with 先攻, which sounds like one specific player or side.

However, Japanese nouns usually do not change form for singular vs. plural, so 友達 can sometimes mean friend or friends depending on context.

Here, the meaning is most likely my friend.

Japanese often leaves out possessives when they are obvious from context.

So 友達 here can naturally be understood as my friend because the speaker is talking about themselves and another participant in the same situation.

Japanese does this a lot:

• 母 can mean my mother
• 先生 can mean the teacher or my teacher
• 友達 can mean my friend

Context does the work.

Yes. It is a complete sentence.

The sentence has two coordinated parts:

• 私は後攻
• 友達は先攻だ

The first part is shortened by omitting だ, but that is completely normal. Japanese often uses this kind of compact structure, especially when listing or contrasting information.

Yes. If you want a polite version, you can say:

私は後攻、友達は先攻です。

or more fully:

私は後攻で、友達は先攻です。

• だ = plain / casual
• です = polite

The meaning stays the same; only the level of politeness changes.