Today I was surfing in Reddit, and found a quote:
The saddest people always try their hardest to make people happy because they know what it’s like to feel absolutely worthless and they don’t want anyone else to feel like that. – Robin Williams
What I wondered was the clause in the sentence: "what it's like to feel worthless". In English, we usually see a clause like what is a dog, in which the word what is the subjective pronoun of the sentence sth is a dog, but the clause mentioned above doesn't seem to be following this rule because it has a subject "it" in it.
I thought that what is probably a determiner for "it" — say, for example "what kind of dog is the biggest breed."— but I found there's a interrogative sentence like "What is it like to be a nurse", so my assumption of what as a determiner seems to be false.
Best Answer
Your confusion may stem from focusing on the wrong word in the sentence. In both examples, "what" has essentially the same meaning or function. It refers to "the identity, nature, or value of an object or matter" (see Webster).
In What is a dog, you are inquiring about the nature of a dog. In ...know what it’s like to... you are referring to the nature of whatever the subject is.
The real difference is inclusion of "like". The meaning of like here is "similar to", although that meaning has to be taken loosely in this usage.
"What is it like to be an astronaut?" means "What is the experience of being an astronaut similar to?" In other words, describe the experience of being an astronaut in terms of something similar that I might be familiar with and can relate to.
This comes into play when you want to talk about an experience, emotion, or something else that is intangible. You can't define or describe it by pointing to it and saying, "It is that." You can only describe it as being similar to something else. It is the difference between, "What is sadness?" (i.e., dictionary definition of the word "sadness"), and "What is it like to feel sad?" (i.e., description of the sensation of experiencing sadness; what is experiencing sadness similar to?).
In your quote:
"knowing what it's like" means they have had a similar experience and know how it feels (as opposed to simply having heard or read about it but not experienced it).