Yes, you do. The sentence is, in a formal sense, "Everybody is happy", and it would be "Everybody's happy" in almost any setting. The apostrophe just abbreviates 'everybody is'. Everybody is singular, which may seem odd at first. Just test it out. Which sounds better: "Everybody are going to the beach!" or, "Everybody is going to the beach!" The word everybody really means every individual, and it is this idea of individual that carries weight in the word. It refers to any people in some group or other, but taken individually rather than together. In other words, any one [of these people] is happy. Another test-- which sounds better "Is everybody happy??" or "Are everybody happy??" The winner is clear, no dispute.
No, you do not need to use an apostrophe in "athletes" when referring to the plural form of the word. An apostrophe is only necessary to indicate possession (e.g., "the athletes' performance"). In general usage, simply writing "athletes" is correct.
after the t (it's) You put an apostrophe on 'its' if it's a contraction of "it is." Otherwise, 'its' has no apostrophe because that's its nature.
Put the apostrophe in mices right after s.: mices'
No apostrophe is to be putted in this sentence.
Personally, no. But it's not incorrect to put the apostrophe.
"Volkswagen" doesn't require an apostrophe.
no matter what you usually have to put a period after an apostrophe because it's the end of a sentence.
Aren't- The apostrophe replaces the 'o' in this circumstance.
won't (:
I'd
No.
we'll