のが／のは

This is beginner stuff that I really want to hammer out in my mind, because I'm still a tad confused. It still freaks me out a little when I see a lot of focuses (wa) in a sentence, and I'm not entirely sure when to use ga or wa.

Dare ga denwa o shimashita ka? Who called? 'Ga' makes sense because you can't focus on and talk about someone you don't know.
Ben-san wa shimashita. Ben called. You can talk about Ben, so wa makes sense.

What I'm confused about is, say...what's the difference between
お寿司が好きです。and
お寿司は好きです。

Or テニスをするのが大好きです！ VS テニスをするのは大好きです！
My only guess is my professor using the case of two 'ni wa' in a sentence, and he said 'wa' was used because of the comparison being done in the sentence. So if I say 'tenisu o suru no wa daisuki desu!' am I saying that I love tennis compared to something else?

よろしくお願いします。

Rei Yahya said:

Dare ga denwa o shimashita ka? Who called? 'Ga' makes sense because you can't focus on and talk about someone you don't know.
Ben-san wa shimashita. Ben called. You can talk about Ben, so wa makes sense.

Click to expand...

I thought you had to use が when people asked to questions like that. が is like, "the one," the person that is doing an action. so, Q: " who (is the one that) called" A: "Ben (is the one that) called" I think anyway.

Rei Yahya said:

What I'm confused about is, say...what's the difference between
お寿司が好きです。and
お寿司は好きです。
Or テニスをするのが大好きです！ VS テニスをするのは大好きです！
My only guess is my professor using the case of two 'ni wa' in a sentence, and he said 'wa' was used because of the comparison being done in the sentence. So if I say 'tenisu o suru no wa daisuki desu!' am I saying that I love tennis compared to something else?
よろしくお願いします。

Click to expand...

I was recently informed on this very website that は is also used as a contrastive marker. So indeed, using は in these sentences means that you are comparing and contrasting it to something.

お寿司は好きです。= I like sushi (as opposed to something else, indeed. Whatever something else is would probably be obvious at the time.)

I assume it's the same for the tennis one.

Alas, I am also a learner. So if I am wrong in any of those points, hopefully someone can correct me. It's also in my interest to know if I am right.

nice explanation lewiig