tomodati ha kaabubooru wo nageru.

は marks 友達 as the topic of the sentence: as for the friend, they throw a curveball.

In a basic sentence like this, は often feels natural when you are simply talking about someone and saying something about them.

If you used が instead, the nuance would be different. が tends to mark the subject more directly and can sound like you are identifying who does the action, often with a sense of focus or contrast.

So:

• 友達はカーブボールを投げる。 = My/The friend throws a curveball / As for my friend, they throw a curveball
• 友達がカーブボールを投げる。 = The friend is the one who throws a curveball

Both can be grammatical, but は is the more neutral topic-marking choice here.

を marks the direct object of the verb.

Here, カーブボール is the thing being thrown, so it gets を:

• カーブボールを投げる = to throw a curveball

A useful pattern to remember is:

• [thing] を [verb]

with を showing what the action is done to.

投げる is the dictionary form and also the non-past form.

In Japanese, the non-past can mean different things depending on context:

• habitual / general action: throws
• future action: will throw

So this sentence could mean:

• A friend throws curveballs
• My friend will throw a curveball

Without more context, Japanese often leaves that open.

Japanese normally puts the verb at the end of the sentence.

So the basic order is often:

• topic + object + verb

In this sentence:

• 友達は = as for the friend
• カーブボールを = curveball
• 投げる = throw

This is very normal Japanese word order.

カーブボール is a loanword, originally from English curveball, so it is written in katakana.

Katakana is commonly used for:

• foreign loanwords
• foreign names
• sound effects
• emphasis in some cases

So seeing カーブボール in katakana tells you it is not a native Japanese word.

By itself, 友達 can mean either friend or friends, depending on context.

Japanese nouns usually do not change form for singular vs. plural the way English nouns do.

So 友達はカーブボールを投げる could be understood as:

• A friend throws a curveball
• My friend throws a curveball
• Friends throw curveballs

Context normally makes it clear.

No. Japanese is normally written without spaces.

A natural written version would be:

友達はカーブボールを投げる。

Spaces are often added in textbooks or beginner materials to make the parts of the sentence easier to see.

Yes. 投げる is a transitive verb here because it takes a direct object, marked by を.

So:

• カーブボールを投げる = throw a curveball

The thing being thrown is the object of the verb.

Yes. Both are common.

• 友達 uses kanji
• 友だち mixes kanji and hiragana

In many cases, both are acceptable, and the meaning is the same. Beginners may also first see it written fully in hiragana as ともだち.

Yes, it often can.

Japanese frequently omits words like my, your, or the when the meaning is clear from context.

So 友達は may naturally be understood as:

• my friend
• a friend
• the friend

depending on the situation.

Japanese often relies more on context than English does.