First things first.
こわい means "to be scary". It's not a verb. It's an i-adjective, which means it describes a state of something, hence the inclusion of "to be".
So 私がこわい means "I'm scary" (to be exact, I think it's: "[Who is scary?] I am the scary one") rather than "I'm scared". That could be: 私はこわい which is using the topic particle は and the so-called "zero-pronoun". But the meaning of 私はこわい doesn't end here.
Let's dig into it a bit.
Most English speakers are hung up on the idea that in every sentence there must be an explicitly stated subject (it's easier on me since my native language is Polish and in Polish the subject-actor is often omitted, although words are inflected in a way that helps us understand WHO is doing something). Japanese tend to omit as much as possible, especially in casual language. That means both subject and topic (which may be the same thing) can be omitted if they can be inferred from context. This concept is difficult to understand for English speakers, since English is a language with a fairly rigid grammar structure. To help English speakers understand that the subject IS there even if sometimes you just don't see it explicitly expressed in the sentence, the concept of the so-called "zero-pronoun" is introduced. And it can be a bit confusing (Personally I don't like it).
Full sentence with こわい, without omissions, would look something like this:
SCARED PERSON は [<=topic] SCARY THING が [<=subject] こわい。
Supposedly.
But the subject can also be the topic:
THING は [<=topic: possibly subject, possibly actor] こわい。
Who is scared in this sentence? What if the scary thing is also a "person" that can be scared"? Does this mean that:
- I'm scared of the THING
- someone I'm talking about is scared of the THING
- the THING is simply objectively scary
- the THING is scared of something
- the THING is scared of itself
All of the above could be correct. It depends on the context: who said it, about what, what was said before this sentence.
Side note: は carries the feeling of "contrast" with it - は contrasts the thing it follows with other things. Something like: "(compered to what I was speaking of before / all the other things) speaking of THIS TOPIC...". In fact, I think this is the primary function of は, and its function as a topic particle stems from exactly this.
So what happens in the above example? How can it mean all these different things?
I'm scared of the THING
If in conversation WHO is not mentioned in the sentence or wasn't explicitly stated as a topic before, it is usually assumed that the speaker is speaking about themselves. In fact, it is considered somehow rude to constantly go 私は this, 私は that, you may sound a bit conceited. Apparently unless Japanese people absolutely need to, they don't say 私は.
So "THING は [<=topic] こわい" can be:
(私 は) THING は こわい。 = (Speaking of me) speaking of this THING (in
contrast to other things), it is scary => I'm scared of this THING.
It isn't that こわい means "to be scared", that's just the way it's translated into English, because it sounds better than the direct translation.
Notice the similarity to "SCARED PERSON は [<=topic] SCARY THING が [<=subject] こわい". It's not that the THING suddenly isn't the subject of this sentence, it's just that the particle が got "swallowed" by the topic particle は. Particle は tends to do that.
someone I'm talking about is scared of the THING
Maybe we're describing someone else at the moment, and that person was mentioned earlier. We know who we're talking about, there's no need to constantly repeat it. For example, I'm talking about Kaori, and she is scared of this thing:
(かおり は) THING は こわい。= (Speaking of Kaori) speaking of this THING, it
is scary => Kaori is scared of the THING.
the THING is simply objectively scary
This one is simple.
THING は こわい。= Speaking of this THING, it is scary. => The THING is
scary.
For example:
クモはこわいですね。= Spiders are scary, aren't they?
the THING is scared of something
Here we can see the power of context.
THING は (some other scary thing が) こわい。= Speaking of this THING, (some
other scary thing) is scary => The THING is scared.
For example 犬はこわい can both mean that "dogs are scary" and "the dog is scared (of something else)".
the THING is scared of itself
This one is interesting. Theoretically (I'd like a confirmation from someone on this)
THING は (THING itself が) こわい。= Speaking of this THING (the THING
itself) is scary => The THING is scared of itself.
An example of something like this (although the が particle is used here): there's a song by Thomas Dolby "I Scare Myself". The title was translated into Japanese as "私がこわい". "I Scare Myself" can technically mean "(to myself) I'm scary"
