Learn Polish: Ten kelner jest bardzo miły.

We use the adjective miły (nice) because it describes the noun kelner (waiter). If we were describing an action or a general state instead of a person, we would use the adverb miło (nicely / nice). For example, if you wanted to say 'it is nice here', you would use the adverb and say Tutaj jest miło.

Why is miły in the Nominative case here? Doesn't the verb być (to be) usually require the Instrumental case?

Great catch! When być links a noun to an adjective (like 'waiter' is 'nice'), the adjective stays in the Nominative case. We only use the Instrumental case when linking a noun to another noun, for example: Ten kelner jest moim bratem (This waiter is my brother).

Why do we use Ten instead of To for 'This'?

Ten is the masculine singular form of 'this', and it has to match the masculine noun kelner. You only use To if the noun is neuter (like to dziecko - this child) or as a general introductory statement (like To jest kelner - This is a waiter).

Does the word bardzo (very) ever change its ending if we talk about a female waitress instead?