using wa or ga. about particles


I started to learn Japanese grammar yesterday,so I am kinda of new to this. My
question is this: in an example in my textbook it says :

> すしは食べた。  
>  Sushi wa tabeta.  
>  I ate sushi. (Literally: As for sushi, I ate it.)

Can I just use "ga" instead of "wa"? When should I use "ga","o" and "wa"?




> すしが食べた。

Would mean that it is the すし that does the action of 食べる. So for this sentence
you cannot use が.

> すしは食べた。

As your textbook says, it means "I ate sushi. (Literally: As for sushi, I ate
it.)" and it gives a nuance of contrast: "As for the sushi, I ate it (, but I
did not eat the other food)".

> すしを食べた。

Is the most neutral way to say "I ate sushi".

If you get asked the question 何を食べましたか。 (what did you eat?), however, you can
only reply by using を (すしを食べました) and not は. The reason for this is that the
question is neutral, so answering it with a sentence that shows a nuance of
contrast such as すしは食べた。 would make it quite unnatural.

That being said, if you get asked the question すしを食べましたか。 (did you eat
sushi?). You could reply by using は, and say something like すしは食べませんでした。

