nowa , to in the phrase "kimi o shiawase ni dekiru nowa boku dake da to shinjite imashita"


I'm trying to make sense of this phrase to me

> 君を幸せにできるのは僕だけだと信じていました。  
>  Kimi o shiawase ni dekiru nowa boku dake da to shinjite imashita.

which I found translated as

> I always believed I was the only one who could make you happy.

"kimi o shiawase ni dekiru" is "be able to make you happy" or something like
that

shinjite imashita is "believed"

"boku dake da" , "I just am" (I suppose)

Question is, is the "who" given by some word or particles (such as nowa) or is
it just the structure of the sentence who puts a "who" when translated there ,
and which meanings/functions have the "nowa" and "to" particles here?




The 'no' in 'nowa' can be thought of as the pronoun 'one'. Your familiar with
phrases like 'aoi **no** ga hoshii desu' = 'I want the blue one' right?

This 'one' pronoun is modified by the relative clause 'kimi o shiawase ni
dekiru'. And then the whole lot is made into a topic with 'wa'. So "The one
who can make you happy" is the topic of the sentence.

'kimi o shiawase ni dekiru' kind of works like the adjective 'aoi' in that it
modifies 'one' and tells you what kind of 'one' the person is talking about.

We could have translated the example as "I want the one **which** is blue", to
which you might legitimately ask 'where did the **which** come from?. It's the
same with 'who' in your example.

If you still can't see where the 'who' comes from try [this
link](http://www.guidetojapanese.org/learn/grammar/clause), or just check out
'relative clauses' in general.

For the last part of your question, the particle 'to' translates to 'that'.It
marks quotes (direct and indirect). "I believed **that** I was the only one
...".




The main (outermost) structure of this sentence is:

> ～と信じていました。  
>  I had believed that ～.

So this と is the plain old quotative particle.

The remaining part (his "belief") is:

> 君を幸せにできるのは僕だけだ。  
>  It is only me who can make you happy.

This の is a nominalizer, but this construction has a special name called a
[cleft sentence](https://japanese.stackexchange.com/a/19208/5010). This is a
typical cleft sentence made from:

> 僕だけが君を幸せにできる。  
>  Only I can make you happy.

That "who" after "one" is called a [relative
pronoun](https://japanese.stackexchange.com/a/14550/5010), and you will not
find its direct equivalent in the Japanese translation. Japanese relative
clauses are much simpler, as you probably know.

