Learn Hardcore Japanese: watasitati ha syuumatu ni tomodati to tanosiku supootu wo simasu. - We happily play sports with friends on the weekend.

It doesn’t mean “wa” by itself; it’s just written with the kana は but pronounced wa when used as a particle.
When は is part of a word (like はな “nose/flower”), it’s pronounced ha. When it’s this grammatical particle, it’s pronounced wa.

Do I have to say 私たち? Can I just say 週末に友達と楽しくスポーツをします?

You can definitely omit 私たち. In natural Japanese, subjects like “I / we / you / they” are often left out when they’re obvious from context.

週末に友達と楽しくスポーツをします。
→ Perfectly natural: “(We/I) have fun playing sports with friends on the weekend.”

You use 私たち when:

You want to emphasize that it’s “we (not someone else)”
The subject isn’t obvious from context

Why is it 週末に and not 週末で? What does に mean here?

Here に is a time marker meaning “at / on (a point in time)”:

週末に = on the weekend / at the weekend

Use に with specific points in time:

月曜日に (on Monday)
3時に (at 3 o’clock)

で is usually used for location of an action:

公園でスポーツをします。
“We play sports at the park.”

So:

週末にスポーツをします。 = We do sports on the weekend (time)
公園でスポーツをします。 = We do sports at the park (place)

Could I drop に and just say 週末、友達と楽しくスポーツをします?

Yes. With some time expressions, に is optional, especially in casual speech:

週末(に), 明日(に), 今週(に) → に can often be omitted.

So:

週末、友達と楽しくスポーツをします。
is also natural and means the same in most contexts. Using に just makes the time marking a bit clearer or more formal.

What does と after 友達 mean in this sentence?

Here と means “with”:

友達と = with (a) friend / with friends

So:

友達と楽しくスポーツをします。
“(We) enjoyably do sports with friends.”

The particle と has several uses (quotations, “and” between nouns, etc.), but after a person + action verb, it often means “together with” that person.

Why is 友達 written in what looks like singular (“friend”), but translated as “friends”?

Japanese usually doesn’t mark plural the way English does. 友達 can mean:

“a friend”
“friends”
“my friend(s)” / “our friend(s)”

The exact number is understood from context or simply left vague.
If you really want to emphasize plural, you can say 友達たち, but that sounds a bit childish or exaggerated in many contexts. Most of the time, just 友達 is fine for “friend(s).”

What exactly is 楽しく? Why isn’t it 楽しい?

楽しい is an -i adjective meaning “fun / enjoyable.”
To make an adverb (“in a fun way / enjoyably”), you change 楽しい → 楽しく.

Pattern:

楽しい (fun) → 楽しく (in a fun way)
早い (fast) → 早く (quickly)

So:

楽しくスポーツをします。
Literally: “(We) do sports enjoyably / in a fun way.”

楽しいスポーツをします would mean something different (see next question).

What’s the difference between 楽しくスポーツをします and 楽しいスポーツをします?

楽しくスポーツをします
- 楽しく = adverb
- Means: “We do sports in a fun way / We have fun playing sports.”
- Focus is on how you do the action.
楽しいスポーツをします
- 楽しい = adjective modifying スポーツ
- Literally: “We do fun sports.”
- Focus is on what kind of sports (fun ones, as opposed to boring or hard ones).

In everyday speech, if you want to say “We have fun playing sports,” 楽しくスポーツをします (or just “スポーツをするのが楽しいです”) is more natural.

Why do we say スポーツをします instead of just using a verb like “play”?

In Japanese, many nouns combine with the verb する (“to do”) to describe actions:

勉強する (to study; literally “do study”)
仕事 をする (to work; “do work”)
スポーツ をする (to play sports; “do sports”)

スポーツ is a noun, and する turns it into an action: “to do sports / to play sports.”

The を marks スポーツ as the direct object of する:

スポーツをします = “(We) do sports / play sports.”

Is を necessary in スポーツをします? Can I say スポーツします?

In conversation, you will often hear スポーツします without を, and it’s understood just fine.

Grammatically:

スポーツをします is the full form (very standard, clear).
スポーツします is a natural, slightly lighter version often used in speech.

Both are acceptable; for beginners and in writing, it’s safer to keep を.

What is the politeness level of します? When would I use する instead?

します = polite form
する = plain (casual/dictionary) form

Use します:

With people you don’t know well
In class, at work, in formal situations
In most textbooks early on

Use する:

With close friends, family (if casual is normal there), peers
In casual writing (messages, some social media, etc.)

So:

Polite: 週末に友達と楽しくスポーツをします。
Casual: 週末に友達と楽しくスポーツをする。

How flexible is the word order? Does it have to be 私たちは週末に友達と楽しくスポーツをします?

Japanese word order is fairly flexible as long as the verb comes at the end and particles stay attached to their words. All of these are grammatical:

週末に私たちは友達と楽しくスポーツをします。
私たちは友達と週末に楽しくスポーツをします。
私たちは友達と楽しく週末にスポーツをします。 (less natural; time usually earlier)

Most natural is close to the original:

私たちは週末に友達と楽しくスポーツをします。

Key rules:

Verb (します) goes at/near the end.
Particles (は, に, と, を) must stay with their nouns/adverbs.