Questions & Answers about Þjónninn sem hjálpar okkur er glaður.
Does the word sem change if we are talking about a female waitress or multiple waiters?
No. sem is an incredibly useful word because it never changes! Whether it means "who", "which", or "that", and regardless of the gender or number of the noun it refers to, it stays exactly the same. In this sentence, it acts as the subject of the relative clause: the one doing the helping.
How can I tell which verb belongs to sem and which belongs to the main sentence?
The verb immediately following sem belongs to the relative clause. In this sentence, hjálpar is the action done by sem (who helps us). The main sentence simply wraps around this extra information: Þjónninn ... er glaður (The waiter ... is happy).
Why do we use okkur for "us" after hjálpar?
The verb hjálpa (to help) is a verb that always requires the object receiving the help to be in the dative case. The dative form of the pronoun við (we) is okkur. (Note: okkur is also the accusative form, but it is strictly the dative being used here because of the rules for hjálpa!).
Why does the sentence use the form glaður for "happy"?