Learn Hardcore Japanese: watasi ha kuruma wo motitai desu.

The actual subject of 持ちたい is also 私, but in Japanese they usually mark the topic with は and then say something about it. が is the prototypical subject marker, but in a simple statement about yourself, 私は is much more natural than 私が.

Why is it 車を and not 車が? I thought が marks the thing that is wanted.

In this pattern, 車 is the direct object of the verb 持つ (to have / to own / to hold), so it takes を.

Base verb: 車を持つ = to have / own a car
Desire form: 車を持ちたい = want to have / own a car

You might be thinking of 車がほしいです, where 車 takes が because:

ほしい is an adjective meaning wanted / desired.
The thing that is wanted by the speaker is marked with が.

So:

車を持ちたいです = I want to have/own a car. (focus on the action/state of possessing)
車がほしいです = I want a car. (focus on the car as something desired)

Both are correct, but the grammar pattern is different.

Does 車を持ちたいです mean I want to hold a car or I want to own a car?

With 持つ, the meaning depends on context, but with 車, it is normally understood as own / have rather than literally hold in your hands (which is physically impossible for a normal car).

ペンを持つ usually = to hold a pen.
車を持つ / 車を持ちたい = to own a car / want to own a car.

So 私は車を持ちたいです is naturally interpreted as:

I want to have my own car / I want to own a car.

Why is it 持ちたい and not 持たい or 持ますたい?

The たい form is made from the ます-stem of the verb:

Take the verb in dictionary form: 持つ.
Find the ます-stem (the form before ます): 持ちます → 持ち.
Add たい: 持ち + たい → 持ちたい.

So:

持たい is wrong (you cannot attach たい directly to the dictionary form 持つ).
持ますたい is wrong (you never keep ます in front of たい).

Correct: 持ちたい.

Is 持ちたい a verb or an adjective? Why can it attach to です?

Grammatically, 〜たい behaves like an い-adjective, even though it’s built from a verb.

Base verb: 持つ
Desire form: 持ちたい (acts like an adjective: want-to-have)

Because 〜たい is an い-adjective, it:

Can be followed by です for politeness: 持ちたいです.
Conjugates like an い-adjective:
- Negative: 持ちたくない (don’t want to have)
- Past: 持ちたかった (wanted to have)
- Polite negative: 持ちたくないです or 持ちたくありません

So 持ちたいです is structurally like saying うれしいです (I’m happy).

Why do we use です at the end? What is the difference between 持ちたい and 持ちたいです?

です adds politeness, not new meaning.

持ちたい = plain / casual form (I want to have).
持ちたいです = polite form (I want to have said politely).

You use 持ちたいです when speaking to people you are not close to, in public situations, at work, etc.

In very casual speech with friends, you would usually just say:

車を持ちたい。
or even add a softening particle: 車を持ちたいな。

Do I have to say 私は? Can I just say 車を持ちたいです?

You can absolutely drop 私は, and that is what Japanese speakers normally do unless they need to be explicit or contrast something.

車を持ちたいです。
→ Naturally understood as I want to have a car from context.

Using 私は:

私は車を持ちたいです。

This is still correct, but it slightly emphasizes me (for example, contrasting with others):

私は車を持ちたいですけど、弟はあまり興味がないです。
→ As for me, I want to have a car, but my younger brother is not very interested.

So in isolation both are fine, but in real conversation you’ll very often hear the version without 私は.

What is the difference between 車を持ちたいです and 車がほしいです?

Both can be translated as I want a car, but there is a nuance difference:

車がほしいです。
- Uses ほしい, an adjective meaning wanted / desired.
- Focus: I want a car (as an object).
- Very common and natural when you just mean you want to have a car.
車を持ちたいです。
- Uses 持つ, to have / own / possess.
- Focus: I want to be in the state of owning a car (having a car as my possession).
- Sounds a bit more like thinking about the lifestyle of owning a car, responsibilities, etc.

In everyday speech, 車がほしいです is more common as a simple statement of desire. 車を持ちたいです is also correct and might sound a bit more deliberate or reflective.

How do I say I don’t want to own a car using this pattern?

Negate the たい part as an い-adjective:

持ちたい → 持ちたくない (don’t want to have)

Then add です for politeness if needed:

私は車を持ちたくないです。
→ I don’t want to own a car.

Casual:

車を持ちたくない。

Can I say 車が持ちたいです?

No, that is not natural and is usually ungrammatical in the intended meaning.

With たい, the subject (the one who wants) is marked with が or は.
The object (the thing you want to do the action on) is marked with を.

So:

私は車を持ちたいです。
→ I want to have a car. (I = subject, car = object)

If you say 車が持ちたいです, it sounds like:

車 is the subject, so literally The car wants to have… (but there is no object).
This is strange and incorrect for your intended meaning.

Stick with 車を持ちたいです.

Can I change the word order, like 車を私は持ちたいです or 車は持ちたいです?

Japanese word order is more flexible than English because of particles, but there are preferred patterns.

私は車を持ちたいです。
- Most neutral and standard: As for me, I want to have a car.
車は持ちたいです。
- 車は makes 車 the topic: As for a car, I want to have (one).
- Implies contrast with other things:
  - 家は要らないけど、車は持ちたいです。
    → I don’t need a house, but I do want a car.
車を私は持ちたいです。
- Grammatically possible but sounds marked / awkward in isolation.
- Could be used for strong emphasis on 私 in a contrastive way in certain contexts, but it’s not the usual pattern.

For basic sentences, keep the normal order: 私は車を持ちたいです or more naturally just 車を持ちたいです.

Is 私は車を持ちたいです a natural thing for Japanese people to say in real life, or is it just a textbook sentence?

It is grammatically correct and usable, but in actual conversation people might phrase the same idea in a few other common ways depending on nuance:

車がほしいです。
→ I want a car. (very common, simple desire)
いつか車を持ちたいです。
→ I want to own a car someday. (adds a time nuance, very natural)
車を買いたいです。
→ I want to buy a car. (focus on the act of buying)
車を持ちたいなあ。 (casual)
→ I really wish I could have a car…

So 私は車を持ちたいです is fine, especially in a textbook or as a clean example of the 〜たい form, but in real speech you’d often:

Drop 私は unless you need it.
Possibly add more context: いつか, 自分の, etc.