We can make a sentence without using any subect after 'what and why' .For example,
What happened? and which is good?.
In 'What happened?' the subject of the verb is 'what'.
In 'Which (one) is good?', the subject of the verb is 'which (one)'.
But can we make like say that these?
How must be said or what for must be said or why must be said?
'What' and 'which' stand for, and are answered by, words representing things, events or situations, which can function as subjects of verbs; 'how',' what for' and 'why' do not.