Do all words need a particle?

Question

I'm trying to figure out if question words need particles. For example

なぜ　(should there be a particle here?) にほんご　を　べんきょうしますか？
Why study Japanese?

Should there be a particle after なぜ? If so which one? If no particle is needed could you say why that is.

Thanks.

Answer 1

You're asking multiple questions.

1. Do all words need a particle?
   A: Of course not. Verbs such as べんきょうします, and many others, appear without a particle.

2. Should there be a particle after なぜ?
   A: If "should" means "is required", then no.

3. If no particle is need, why?
   A: It's difficult to give the reason why something is not the case. Particles fulfill certain grammatical functions, such as marking case, syntactic relationship, etc., or pragmatic functions, such as speech acts (questions, requests, etc.), or illocutionary force (～よ, ～ね, etc.), etc. なぜ is an interrogative adverb, and hence appears without a particle.

Also note that some, but not all, particles can be dropped in casual speech.