tomodati ha kaabubooru wo nagetagatte iru.

は marks the topic of the sentence. It tells you that the sentence is about 友達.

In a simple sentence like this, the topic is also understood as the person who has the desire, so 友達 is the one who wants to throw the curveball.

を marks the direct object of the verb. In other words, it marks the thing being thrown.

Here:

• カーブボール = the thing
• を = object marker
• 投げる = to throw

So カーブボールを投げる means to throw a curveball.

No. Here, た is part of the grammar pattern 〜たがる, not the past tense ending.

Compare:

• 投げた = threw
• 投げたがる = shows signs of wanting to throw

So this sentence is not in the past. If it were past, you would see something like 投げたがっていた.

It is built like this:

• 投げる = to throw
• verb stem: 投げ-
• たがる = to show signs of wanting to do something
• ている = ongoing state

So:

投げる → 投げたがる → 投げたがっている

The full form means something like is showing that they want to throw.

〜たい is normally used for your own desire:

• カーブボールを投げたい = I want to throw a curveball

For a third person, Japanese usually uses 〜たがる / 〜たがっている instead, because the speaker is describing visible signs of desire rather than directly stating someone else’s inner feelings.

So:

• 友達はカーブボールを投げたがっている = My friend seems to want to throw a curveball

In natural English, this is often translated simply as My friend wants to throw a curveball.

Here, ている marks an ongoing state, not an action in progress.

So 投げたがっている does not mean the friend is currently in the middle of throwing. It means the friend is currently showing that desire.

A natural way to think of it is:

• is wanting to throw
• is acting like they want to throw
• seems eager to throw

Usually not in a normal, direct statement.

If you want to say I want to throw a curveball, the natural Japanese is:

• 私はカーブボールを投げたい

Using 〜たがっている for yourself sounds unusual, because that pattern is mainly used to describe someone else’s observable desire.

It can mean either. Japanese nouns usually do not change form for singular vs. plural.

So 友達 can mean:

• friend
• friends

In this sentence, context would decide. If there is no extra context, many learners would take it as singular.

Japanese often leaves that kind of information unstated.

So 友達 could be understood as:

• my friend
• a friend
• the friend

English requires a choice, but Japanese often lets context decide.

They are usually added only for learners.

Normal Japanese is normally written without spaces:

友達はカーブボールを投げたがっている。

The spaced version is just there to make the parts easier to see.