Learn Italian: I reliquiari sono sul tavolo. - The reliquaries are on the table.

For masculine nouns ending in -io, the spelling of the plural depends on where the stress falls in the singular word. In reliquiario, the stress falls on the a, leaving the i unstressed. When the i is unstressed, the plural drops the final o, leaving a single i (reliquiari). You only use a double ii if the stress falls on the i itself, like in zio (uncle), which becomes zii.

How can I tell if the i in an -io noun is stressed so I know whether to use one i or two in the plural?

Italian spelling usually doesn't mark the stress unless it falls on the very last letter, so you have to learn the pronunciation when you learn the word. However, as a helpful shortcut, the vast majority of -io nouns have an unstressed i and take a single i in the plural. Plurals with a double ii are mostly found in a few short words like zio (uncle, plural zii) or addio (farewell, plural addii).

Why use tavolo instead of tavola in this sentence?