I was always told that apostrophes are used to show ownership. So why is it not correct in this sentence: "The dog did not like it's bath."?
For example, you wouldn't say "We did not like our's bath". It's the same for "The dog didn't like its bath."
"It" is dervied from the Old English word "hit" which is the neuter version of the pronoun "he".
Whilst we use apostrophes to show ownership with nouns such as "the soldier's uniform" or "father's car", we don't use them with pronouns. We would say "his uniform" not "him's uniform".
Therefore, when we are using pronouns we would say "his" "hers" or "its" (not "her's" nor "it's").
Note, however, that it is acceptable to use "it's" as a short form of "it is" (i.e. "it's hot today").