Ok…Hi there!
When I say, “They don’t know how to dance”, I’m sure that the infinitive with how is ok. But I haven’t been able to come up with the usage for the inifnitve with who in a sentence.
I don’t know if this is o.k. “I haven’t decided who to invite to the party” Even so, is it common to use the full infinitive after the WH-question who? Because for me it’s much better to say, “I haven’t decided who I should invite to the party”
Both are correct!
The difference is the added word “should”.
In “who to invite to the party”, there is some sense of obligation contained in the infinitive. This is the same sense as is conveyed by “should” in “who I should invite to the party”.
Infinitives with “to” may have different shades of meaning in them, depending on context.
I want to dance. (No extra shade of meaning.)
I don’t know what to do. (Equivalent of “…what I should do”: extra sense of obligation or “what it is best to do”. I think this applies to all indirect questions.)
Did you find any enemies to kill? (Equivalent of “…that could be killed”: extra sense of possibility, potentiality. The sense of the infinitive is also arguably passive here: the enemies are the ones that could be killed, not the ones that could kill. I think this shade of meaning comes to all infinitives that are attached as appositions to nouns or adjectives.)