What would happen if I use は or が in 今日何を食べたいですか?


> 今日何を食べたいですか  
> What do you want to eat today?

In other words, what would happen if I say:

> 今日何は食べたいですか ?
>
> 今日何が食べたいですか ?

(I want to know the rough meaning of the sentence even if the sentence is
wrong.)




You can say...

> 今日は何 **を** 食べたいですか？  
>  今日は何 **が** 食べたいですか？

を is the object marker. You can use が here instead, since が can mark the
object of the ～たい form verb.

Your second sentence:

> 今日何 **は** 食べたいですか？

would be incorrect. You don't use the topic/contrastive は for a question word.  
◎「何が～～？」 ◎「誰が～～？」 ◎「いつ(が)～～？」etc.  
×「何は～～？」 ×「誰は～～？」 ×「いつは～～？」 etc.

As you know, は is basically the topical/thematic particle (主題の「は」) or the
contrastive particle (対比の「は」), and can replace が, を. But it can't be attached
to the question word 何、どこ、誰、いつ etc... (cos 何、どこ etc. can't be a topic/theme of
a sentence, nor can they be contrasted with some other element of the
sentence.)




You can say...

> 今日は何 **を** 食べたいですか？  
>  今日は何 **が** 食べたいですか？

を is the object marker. You can use が here instead, since が can mark the
object of the ～たい form verb.

Your second sentence:

> 今日何 **は** 食べたいですか？

would be incorrect. You don't use the topic/contrastive は for a question word.  
◎「何が～～？」 ◎「誰が～～？」 ◎「いつ(が)～～？」etc.  
×「何は～～？」 ×「誰は～～？」 ×「いつは～～？」 etc.

As you know, は is basically the topical/thematic particle (主題の「は」) or the
contrastive particle (対比の「は」), and can replace が, を. But it can't be attached
to the question word 何、どこ、誰、いつ etc... (cos 何、どこ etc. can't be a topic/theme of
a sentence, nor can they be contrasted with some other element of the
sentence.)

