Is it always necessary to have the definite article in sentences with relative clauses:
The waitress who served us was awesome.
Thank you for your help!
in sentences with relative clauses
"A relative clause—also called an adjective or adjectival clause" "It is sometimes called an “adjective clause” because it functions like an adjective"
Nearly all the time you may remove an attributive adjective from a sentence and it still makes sense grammatically.
When analyzing grammar, this can be helpful.
Removing the relative clause:
The waitress was awesome.
It's fundamentally the same sentence. Now ask, is the article necessary? In other words, could it be
Waitress was awesome. *
Only if speaking Russian. :-) Not in English where articles are quite common and necessary. In any case, the presence or absence of the relative clause doesn't affect the decision here.
Not at all. Because 'the' highlights the restrictive or defining function of relative clauses most clearly, examples of this type tend to be discussed to the exclusion of all others.
But there are other cases where relative clauses do not require the definite article. For example:
Any person who is a friend of Bill's is a friend of mine.
A waiter who can't wait on five tables at the same time is not worth his salt.
A girl who he'd met just the week before moved into his apartment yesterday.
In all these cases the relative clause is defining the person (or type of person) in question without requiring the definite article.
The first example uses the quantifier 'any' to cover an entire set of people defined by the relative clause. The second is similar.
The third is 'specific' but not 'definite'. To be definite, the girl in question must be known to the listener. In this example, the speaker has a specific girl in mind, but 'the' cannot be used because the speaker has only just introduced her into the conversation.