Why do we need multiple words that mean essentially the same thing? We don't have an answer for that, but we can help with your somebody and someone and nobody and no one dilemma.
Let’s break down the difference between each pair, along with some examples and mnemonic devices to help you remember what word to use. By the end, nobody will feel left behind and at least someone will feel good about when to use each word.
Somebody is an indefinite pronoun. It refers to an unnamed, unspecified person. The name of the person might be known, but it is not relevant to the situation or sentence. Nor is it given. If you are living in a home with several people, you might say, “Did somebody lock the door yet tonight?” You know the names of the people who live with you. But it isn’t important who locked the door. It’s just important that it was done by somebody.
Someone is also an indefinite pronoun, but it has a slightly different use. Someone is considered to be slightly more formal than somebody. If you were writing a term paper, or a professional email, it would make more sense to use the word someone over somebody.
A helpful trick to remember which word to use is that someone has an “n” in it, as does the word professional. Somebody has no “n,” and it doesn’t need to be used in more professional settings. Aside from the formal/informal connotations of the two words, they can be used interchangeably.
The distinction between nobody and no one is the exact same type of situation. The two are synonyms with slight nuances as to the distinction between the words.
Nobody is a pronoun, just like somebody, and it means not any person. Although nobody is generally singular, it is also sometimes used as a plural indefinite pronoun, which just means that it doesn’t specify whether plural or singular, and it doesn’t indicate gender.
Example: “Even though I planned my party months in advance and sent out invites three months ahead of time, nobody has RSVPed yet.”
You could easily substitute “no one” into the above sentence and it would not change the meaning aside from indicating that the setting for the sentence was a more formal one.
Bottom line: if you are writing a paper for a professor or a formal letter, stick with no one and some one. However, if you are not in a formal setting, either will do. Also, the same can be said for anyone/anybody and everyone/everybody. In all of the instances, the words that have “body” in them are the more informal words to use.