Are you talking about "The" in "The slightly deranged villain"? "villain" is a singular countable noun. It needs an article or other determiner. The person writing this example sentence chose to use "the".
"the" before sentence, which is what you wrote, looks like a faulty attempt to say "the" at the start of the sentence. Though I should have realised anyway, what you actually meant was "the" before "sentence".
"phrases and clauses building blocks of the sentence" is not correct. It should be something like "Phrases and clauses — building blocks of the sentence".